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: I12ee579625ee6ba493b970601b62f14614ce8658
c61739dd38