Mount kdump nfs targets before trying to use them

Kdump over NFS won't even try anymore if it's not mounted. See
the discussion in rhbz#808298

Mount it before we try to set it as a target so kdump can deal
with it, then ummount it if it's unset. Mount it in ovirt-kdump

Change-Id: I7b64094fe1c42638eac00ad81c79c043b884a528
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1095140
Signed-off-by: Ryan Barry <rbarry@redhat.com>