Install MySQL-python for keystone with rpm system

MySQL-python is needed for keystone and we were installing it
implicitely only for apts and not for rpms. Fix keystone only devstack
install on a pristine vm.

Change-Id: I0d117513af4e2ad58635f7a7b22f7a6e3ff36a38
Merge "Install MySQL-python for keystone with rpm system"