rgmanager: Add IP resource agent "prefer_interface" parameter

This patch adds a "prefer_interface" parameter for IP resources.
The interface must already be configured and active. This
parameter should be used only when at least two active interfaces
have IP addresses on the same subnet and it's necessary to specify
which particular interface should be used.

Acked-by: Lon Hohberger <lhh@redhat.com>
Signed-off-by: Ryan McCabe <rmccabe@redhat.com>