[SQLalchemy] Add groupby ability resource_metadata

It would be useful to have groupby functionality to derive meters
by their resource_metadata. This commit adds groupby by
resource_metadata.instance_type for sqlalchemy.

Change-Id: I568b5ca3d84371c4fd3bbdfed7b3e801164dbc5e
Partially-Closes-Bug: 1331508
Revert "Skip to poll and publish when no resources found"

We revert commit 934a90d4c61ff66f99e5a46480047dec355b5d61
as it doesn't take into account that pollsters may have no
expectation of an explicit resource list, such as in the
IPMI case.

Change-Id: Ib9579417cd0654b790cc0b3c111b06e4dcecf728
Closes-Bug: #1403280
Merge "[SQLalchemy] Add groupby ability resource_metadata"