Handling the cases for UNSET_VALUE, i.e., config is not finalized.

plugins/provisioners/ansible/cap/guest/pip/pip.rb

@@ -25,9 +25,10 @@ module VagrantPlugins
             # The objective here is to get pip either by default
             # or by the argument passed in. The objective is not 
             # to circumvent the pip setup by passing in nothing.
-            # Thus, we stick with the default on an empty string.
+            # Thus, we stick with the default on an empty string
+            # or if it is an UNSET_VALUE.
 
-            if pip_install_cmd.empty?
+            if pip_install_cmd == Vagrant.plugin("2", :config)::UNSET_VALUE || pip_install_cmd.empty?
               pip_install_cmd=DEFAULT_PIP_INSTALL_CMD
             end
 

test/unit/plugins/provisioners/ansible/cap/guest/pip/pip_test.rb

@@ -42,6 +42,11 @@ describe VagrantPlugins::Ansible::Cap::Guest::Pip do
             expect(communicator).to receive(:execute).with("curl https://bootstrap.pypa.io/get-pip.py | sudo python")
             subject.get_pip(machine,pip_install_command)
         end 
+        
+        it "installs pip using the default command if the argument is UNSET_VALUE" do
+            expect(communicator).to receive(:execute).with("curl https://bootstrap.pypa.io/get-pip.py | sudo python")
+            subject.get_pip(machine, Vagrant.plugin("2", :config)::UNSET_VALUE)
+        end         
     end    
   end
 end