Bug 21073: Restore filtering by metadata

Bug 21073: Restore filtering by metadata

This patch restores filtering the plugins by metadata. That got lost on
rebase at some point. Regression tests are added on a prior patch.

To test:
- Have the 'regression tests for GetPlugins' patch applied
- Run:
  $ kshell
 k$ prove t/db_dependent/Plugins.t
=> FAIL: Tests fail!
- Apply this patch
- Run:
 k$ prove t/db_dependent/Plugins.t
=> SUCCESS: Tests pass!
- Sign off :-D

Signed-off-by: Agustin Moyano <agustinmoyano@theke.io>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>