Adds Warning when NetworkManager is active on hosts

Fixes: rhbz#1117115
Signed-off-by: Gael Chamoulaud <gchamoul@redhat.com>
(cherry picked from commit 2faa9a2cb616615f189a7ed6a7eade9a9f12c0e6)
(cherry picked from commit ff7caad9f1943d3d09f511efad8a1dcbb6a165c7)

Conflicts:
	packstack/plugins/neutron_350.py

Change-Id: I7e0fed8b47dabb492f2f3e683f8e8d65497ad816