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
Remove duplicate options(joinedload) from aggregates db code

_aggregate_get_query already does option joinedload.  So when use it don't
need to use additional options(joinedload).

Change-Id: I11a080c262122bf1b59984e5e80f6095527bf507
Merge "Shrink size of aggregate_metadata_get_by_host sql query"
Merge "Remove duplicate options(joinedload) from aggregates db code"