Switch the repo_server role to master
Switch the repo_server role to master

Due to bug 1632847 the repo_server role wa sheld back
in https://review.openstack.org/380325

With https://review.openstack.org/385625 the issues
appear to have been resolved, or at least worked around.

This patch implements testing for this role directly
from the master branch instead of a pinned SHA.

Change-Id: Icf2f00c57fd4b698b15225a4d06eee3d7e2bcd1c
Related-Bug: #1632847

openstack/openstack-ansible,weezer/openstack-ansible,Logan2211/openstack-ansible,Logan2211/openstack-ansible,BjoernT/openstack-ansible,cloudnull/os-ansible-deployment,cloudnull/os-ansible-deployment,robb-romans/openstack-ansible,stackforge/os-ansible-deployment,BjoernT/os-ansible-deployment,weezer/openstack-ansible,cfarquhar/openstack-ansible,bunchc/openstack-ansible,bunchc/openstack-ansible,os-cloud/os-ansible-deployment,cfarquhar/openstack-ansible,BjoernT/os-ansible-deployment,yanyao/openstack-ansible,yanyao/openstack-ansible,BjoernT/openstack-ansible,openstack/openstack-ansible,robb-romans/openstack-ansible,os-cloud/os-ansible-deployment,stackforge/os-ansible-deployment,BjoernT/os-ansible-deployment