Eager load snapshot_metadata in *snapshot_get_all

Eager load snapshot_metadata in *snapshot_get_all

All methods returning snapshot lists in db.sqlalchemy.api are eager
loading snapshot_metadata - besides snapshot_get_all_by_project and
snapshot_get_active_by_window. In case of the latter that fact caused
unit tests to randomly fail because of SQLAlchemy Session sometimes
getting closed before the metadata got lazy loaded. This commit adds
missing options(joinedload('snapshot_metadata')) to these queries.

Change-Id: I72557ebc7af9f3044046965ca79c9fe7c15520fc
Closes-Bug: 1501838