tests: Add missing pip_install role dependency
tests: Add missing pip_install role dependency

This fixes the following problem:

ERROR! the role 'pip_install' was not found in
/vagrant/tests/common/roles:/home/vagrant/.ansible/roles:/home/vagrant/.ansible/testing-role:/vagrant/tests/common
The error appears to have been in '/vagrant/tasks/main.yml': line 74,
column 11, but may be elsewhere in the file depending on the exact
syntax problem.

The offending line appears to be:
  include_role:
    name: pip_install
          ^ here
        to retry, use: --limit @/vagrant/tests/test.retry

Change-Id: I960e9c4e0e060dbbd65ecd2fee5e56d394fe6e88

openstack/openstack-ansible-openstack_hosts,openstack/openstack-ansible-openstack_hosts