Revert "Remove pip_install dependency"

This reverts commit 0aa4981305.
The repo_server and repo_build shouldn't assume to be configured
by openstack_hosts role yet, because their environment is incomplete.

On top of that, when artifacting will be introduced, these roles could
be running outside the openstack hosts (for example, on deploy nodes).

Change-Id: I47a58a46a695b504646bca8100758decb07cbb98

doc/source/index.rst

@@ -14,10 +14,10 @@ Default variables
    :language: yaml
    :start-after: under the License.
 
-Dependencies
-~~~~~~~~~~~~
+Required variables
+~~~~~~~~~~~~~~~~~~
 
-This role needs pip >= 7.1 installed on the target host.
+None.
 
 Example playbook
 ~~~~~~~~~~~~~~~~

meta/main.yml

@@ -38,3 +38,4 @@ dependencies:
   - role: apt_package_pinning
     when:
       - ansible_pkg_mgr == 'apt'
+  - role: pip_install