provisioners/ansible: provide ssh identities via ANSIBLE_SSH_ARGS (when necessary)

When provisioning multiple machines in sequence (the default vagrant
behaviour), it doesn't make sense to require to provide the private ssh
key(s) via the custom ansible inventory script/file.

To align with the handling of multiple ssh keys per machine, we won't
rely any longer on `--private-key` command line argument, but only pass
the keys via `ANSIBLE_SSH_ARGS` environment variable.

Note that when vagrant generates the ansible inventory and that only one
key is associated to a VM, this step would be redundant, and therefore
won't be applied.

This change fixes the breaking change introduced by 3d62a91.

plugins/provisioners/ansible/provisioner.rb

@@ -242,8 +242,10 @@ module VagrantPlugins
         ssh_options << "-o IdentitiesOnly=yes" unless Vagrant::Util::Platform.solaris?
 
         # Multiple Private Keys
-        @ssh_info[:private_key_path].drop(1).each do |key|
+        unless !config.inventory_path && @ssh_info[:private_key_path].size == 1
-          ssh_options << "-o IdentityFile=#{key}"
+          @ssh_info[:private_key_path].each do |key|
+            ssh_options << "-o IdentityFile=#{key}"
+          end
         end
 
         # SSH Forwarding

test/unit/plugins/provisioners/ansible/provisioner_test.rb

@@ -513,14 +513,14 @@ VF
         config.skip_tags = %w(foo bar)
         config.limit = 'machine*:&vagrant:!that_one'
         config.start_at_task = 'an awesome task'
-        config.raw_arguments = ["--why-not", "--su-user=foot", "--ask-su-pass", "--limit=all"]
+        config.raw_arguments = ["--why-not", "--su-user=foot", "--ask-su-pass", "--limit=all", "--private-key=./myself.key"]
 
         # environment variables
         config.host_key_checking = true
         config.raw_ssh_args = ['-o ControlMaster=no']
       end
 
-      it_should_set_arguments_and_environment_variables 20, 4, true
+      it_should_set_arguments_and_environment_variables 21, 4, true
       it_should_explicitly_enable_ansible_ssh_control_persist_defaults
       it_should_set_optional_arguments({  "extra_vars"          => "--extra-vars=@#{File.expand_path(__FILE__)}",
                                           "sudo"                => "--sudo",
@@ -537,15 +537,17 @@ VF
 
       it "also includes given raw arguments" do
         expect(Vagrant::Util::Subprocess).to receive(:execute).with { |*args|
+          expect(args).to include("--why-not")
           expect(args).to include("--su-user=foot")
           expect(args).to include("--ask-su-pass")
-          expect(args).to include("--why-not")
+          expect(args).to include("--limit=all")
+          expect(args).to include("--private-key=./myself.key")
         }
       end
 
       it "shows the ansible-playbook command, with additional quotes when required" do
         expect(machine.env.ui).to receive(:detail).with { |full_command|
-          expect(full_command).to eq("PYTHONUNBUFFERED=1 ANSIBLE_HOST_KEY_CHECKING=true ANSIBLE_FORCE_COLOR=true ANSIBLE_SSH_ARGS='-o IdentitiesOnly=yes -o IdentityFile=/my/key2 -o ForwardAgent=yes -o ControlMaster=no -o ControlMaster=auto -o ControlPersist=60s' ansible-playbook --user=testuser --connection=ssh --timeout=30 --limit='machine*:&vagrant:!that_one' --inventory-file=#{generated_inventory_dir} --extra-vars=@#{File.expand_path(__FILE__)} --sudo --sudo-user=deployer -vvv --ask-sudo-pass --ask-vault-pass --vault-password-file=#{File.expand_path(__FILE__)} --tags=db,www --skip-tags=foo,bar --start-at-task='an awesome task' --why-not --su-user=foot --ask-su-pass --limit='all' playbook.yml")
+          expect(full_command).to eq("PYTHONUNBUFFERED=1 ANSIBLE_HOST_KEY_CHECKING=true ANSIBLE_FORCE_COLOR=true ANSIBLE_SSH_ARGS='-o IdentitiesOnly=yes -o IdentityFile=/my/key1 -o IdentityFile=/my/key2 -o ForwardAgent=yes -o ControlMaster=no -o ControlMaster=auto -o ControlPersist=60s' ansible-playbook --user=testuser --connection=ssh --timeout=30 --limit='machine*:&vagrant:!that_one' --inventory-file=#{generated_inventory_dir} --extra-vars=@#{File.expand_path(__FILE__)} --sudo --sudo-user=deployer -vvv --ask-sudo-pass --ask-vault-pass --vault-password-file=#{File.expand_path(__FILE__)} --tags=db,www --skip-tags=foo,bar --start-at-task='an awesome task' --why-not --su-user=foot --ask-su-pass --limit='all' --private-key=./myself.key playbook.yml")
         }
       end
     end