Remove aggregate metadata methods from conductor and virtapi

The aggregate_metadata_add() and aggregate_metadata_remove()
methods are no longer needed in virtapi and conductor. This
patch removes them from the former and deprecates them in the
latter. Also, aggregate_get_by_host() is no longer needed in
virtapi, so remove it from there.

Related to blueprint compute-manager-objects

Change-Id: Ia057dcc43c555187a4a1466a9e748b978c46d4a4
Merge "Remove aggregate metadata methods from conductor and virtapi"