[minor] Use the correct "pip_install_cmd" name

There is no such thing like 'pip_install_command' option in the
Ansible Local provision, so let's avoid any misunderstanding ;-)

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

@@ -23,30 +23,30 @@ describe VagrantPlugins::Ansible::Cap::Guest::Pip do
   end
 
   describe "#get_pip" do
-    describe 'when no pip_install_command argument is provided' do
+    describe 'when no pip_install_cmd argument is provided' do
       it "installs pip using the default command" do
         expect(communicator).to receive(:execute).with("curl https://bootstrap.pypa.io/get-pip.py | sudo python")
         subject.get_pip(machine)
       end
     end
 
-    describe 'when pip_install_command argument is provided' do
+    describe 'when pip_install_cmd argument is provided' do
       it "runs the supplied argument instead of default" do
-        pip_install_command = "foo"
-        expect(communicator).to receive(:execute).with(pip_install_command)
-        subject.get_pip(machine,pip_install_command)
+        pip_install_cmd = "foo"
+        expect(communicator).to receive(:execute).with(pip_install_cmd)
+        subject.get_pip(machine, pip_install_cmd)
       end
 
       it "installs pip using the default command if the argument is empty" do
-        pip_install_command = ""
+        pip_install_cmd = ""
         expect(communicator).to receive(:execute).with("curl https://bootstrap.pypa.io/get-pip.py | sudo python")
-        subject.get_pip(machine,pip_install_command)
-      end 
-        
+        subject.get_pip(machine, pip_install_cmd)
+      end
+
       it "installs pip using the default command if the argument is nil" do
         expect(communicator).to receive(:execute).with("curl https://bootstrap.pypa.io/get-pip.py | sudo python")
         subject.get_pip(machine, nil)
-      end         
-    end    
+      end
+    end
   end
 end