Add openstack-rally-plugins project
Add openstack-rally-plugins project

It contains all the rally plugins which can be consumed as a deps
of rally project.

Change-Id: I8253e4820d2ee057d18cac6cc7515b5c5561dd39
BZ: https://bugzilla.redhat.com/show_bug.cgi?id=1615913
Tag: under-review

redhat-openstack/rdoinfo,javierpena/rdoinfo