Shrink size of aggregate_metadata_get_by_host sql query

* Remove unnecessary secondary joins in Aggregate model.
* Remove unnecessary backrefs (only needed to create bidirectional relationship)

aggregate_metadata_get_by_host goes from 6 JOINS down to two JOINS.

Fix bug 1133495

Change-Id: I72966fa205973ec638816b07bfdcd54f1102feb5
(cherry picked from commit ce23d6c1d679355bafede816bca39ea1eb24073c)
Merge "Shrink size of aggregate_metadata_get_by_host sql query" into stable/folsom