engine: Improve messages in scheduling

Add a way of adding more detailed explanations to the host
was filtered out messages.

Also change the message for the situation where no host
was available from the beginning.

Change-Id: I0d3c4d50c3a7a8cd11b46b19c1167daace8461be
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1077627
Signed-off-by: Martin Sivak <msivak@redhat.com>