Remove join filter from optional pip module tasks

Ansible 2.2 now treats the 'name' argument for the pip module
as a list, removing the need for us to implement the join
filter to optimise the install execution.

Change-Id: I2d271bf3e822c86468814a62a761d665aab30d78

tasks/nova_compute_ironic.yml

@@ -15,7 +15,7 @@
 
 - name: Install pip packages (venv)
   pip:
-    name: "{{ nova_compute_ironic_pip_packages | join(' ') }}"
+    name: "{{ nova_compute_ironic_pip_packages }}"
     state: "{{ nova_pip_package_state }}"
     virtualenv: "{{ nova_bin | dirname }}"
     virtualenv_site_packages: "no"