Optimise pip install tasks

Unlike the Ansible apt module, the Ansible pip module does not
recognise a with_items list and process all the items at once.

To optimise the pip install tasks, this patch replaces the use
of with_items with a join filter so that the pip install task
does an install with all the packages in a list, ensuring that
the execution is one action instead of many.

Change-Id: I202b576df32607c23bdcd1a5d23245c0660a0758

tasks/galera_client_install.yml

@@ -27,13 +27,12 @@
 
 - name: Install pip packages
   pip:
-    name: "{{ item }}"
+    name: "{{ galera_client_pip_packages | join(' ') }}"
     state: latest
     extra_args: "{{ pip_install_options|default('') }}"
   register: install_packages
   until: install_packages|success
   retries: 5
   delay: 2
-  with_items: "{{ galera_client_pip_packages }}"
   tags:
     - galera-client-pip-packages