fix `config.ssh.keys_only = false` for ansible

Also see #5017.

plugins/provisioners/ansible/provisioner/host.rb

@@ -249,11 +249,8 @@ module VagrantPlugins
           # Don't access user's known_hosts file, except when host_key_checking is enabled.
           ssh_options << "-o UserKnownHostsFile=/dev/null" unless config.host_key_checking
 
-          # Set IdentitiesOnly=yes to avoid authentication errors when the host has more than 5 ssh keys.
-          # Notes:
-          #  - Solaris/OpenSolaris/Illumos uses SunSSH which doesn't support the IdentitiesOnly option.
-          #  - this could be improved by sharing logic with lib/vagrant/util/ssh.rb
-          ssh_options << "-o IdentitiesOnly=yes" unless Vagrant::Util::Platform.solaris?
+          # Compare to lib/vagrant/util/ssh.rb
+          ssh_options << "-o IdentitiesOnly=yes" if !Vagrant::Util::Platform.solaris? && @ssh_info[:keys_only]
 
           # Multiple Private Keys
           unless !config.inventory_path && @ssh_info[:private_key_path].size == 1

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

@@ -40,6 +40,7 @@ VF
   let(:config)  { VagrantPlugins::Ansible::Config::Host.new }
   let(:ssh_info) {{
     private_key_path: ['/path/to/my/key'],
+    keys_only: true,
     username: 'testuser',
     host: '127.0.0.1',
     port: 2223
@@ -988,5 +989,20 @@ VF
       end
 
     end
+
+    describe 'with config.ssh.keys_only = false' do
+      it 'does not set IdentitiesOnly=yes in ANSIBLE_SSH_ARGS' do
+        ssh_info[:keys_only] = false
+
+        expect(Vagrant::Util::Subprocess).to receive(:execute).with { |*args|
+          cmd_opts = args.last
+          expect(cmd_opts[:env]['ANSIBLE_SSH_ARGS']).to_not include("-o IdentitiesOnly=yes")
+
+          # Ending this block with a negative expectation (to_not / not_to)
+          # would lead to a failure of the above expectation.
+          true
+        }
+      end
+    end
   end
 end