Remove join filter from pre-build 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: I002e0cacaccc33acd3590baf1a45a61de52afe25

tasks/repo_pre_build.yml

@@ -73,7 +73,7 @@
 
 - name: Install pip packages
   pip:
-    name: "{{ repo_pip_packages | join(' ') }}"
+    name: "{{ repo_pip_packages }}"
     state: "{{ repo_build_pip_package_state }}"
     extra_args: "--constraint {{ repo_build_release_path }}/{{ repo_build_release_tag }}/requirements_constraints.txt {{ pip_install_options }}"
   register: install_packages