Fix NetworkManager facter error

When puppet is run with NetworkManager disabled it gets an error
from facter that is cascaded to packstack causing a failure.

Facter (>= 2.1.0) ships a new structured facts called dhcp_servers which
returns a hash of DHCP Server addresses. That fact requires NetworkManager
running (https://tickets.puppetlabs.com/browse/FACT-697).

Fixes: rhbz#1116403

Change-Id: Ied08f797dd9ea5039e3d03d42eb172c77ab4113a