vdsm: Reduce the migration progress timeout

The progress of the migration should indicate a stuck migration
way before the currently configured migration_timeout, which has
by default 300 seconds.

Half of the time should be more than enough for now.

This commit introduces the migration_progress_timeout configuration
value to be able adjusting this value.

Change-Id: I8f314d70b8f32cfff58f9776bcc2182a748a9b67
Signed-off-by: Vinzenz Feenstra <vfeenstr@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/16602
Reviewed-by: Dan Kenigsberg <danken@redhat.com>