gluster: Return UNKNOWN status for GlusterTaskStatus

If the string value passed to GlusterTaskStatus is not
one of the enum options return UNKNOWN as the option
value.
Fixes 2 issues reported by coverity scan when converting
vdsm return value
GlusterAsyncTaskStatus.from((String)map.get(STATUS)).getJobExecutionStatus()
-- possible NPE

Change-Id: If8cd725af21639827ba19c5589fe2889e1c98c19
Signed-off-by: Sahina Bose <sabose@redhat.com>