enable multiple role assignment.
case neutron-l3 is assigned before neutron-server.

(cherry picked from commit c447fa30d5669be67281b58ae946df172f61cd90)
Removed the single assignment block from constraints

(cherry picked from commit 48d52521546413b98fe321efcf4beade49c65d53)
Merge pull request #60 from vuntz/release/stoney/master

Sync with roxy