os: do not attempt to detect discard by default

If the journal_discard option is false, the journal should no try to
guess if the device has discard capabilities.

http://tracker.ceph.com/issues/10296 Refs: #10296

Signed-off-by: Loic Dachary <ldachary@redhat.com>
tests: do not ignore ceph-disk.sh::test_activate_dev failure

The status of the test commands must be tested instead of being
silently ignored.

Signed-off-by: Loic Dachary <ldachary@redhat.com>
Merge pull request #3152 from dachary/wip-10296-journal-discard

os: do not attempt to detect discard by default

Reviewed-by: Sage Weil <sage@redhat.com>