Improve messages on node state transitions
Improve messages on node state transitions

First, the state is called 'manageable', not 'managed'. Second,
the current messages do not provide enough context to be valuable.
This change modifies them to include nodes count and uses more
precise wording.

Change-Id: Ia1226123271408e959d7aa2049140c06f7ba5e87

openstack/tripleo-common,openstack/tripleo-common