Fix a usage error of joinedload + filter in l3 scheduler

Fix a usage error of joinedload + filter in l3 scheduler

This commit fixes admin_state_up filtering in
get_l3_agents_hosting_routers.  Also, adapt its callers
which rely on the current broken implementation.

Details:

With the current coding, joinedload() produces a JOIN and
the following filter() on the columns from the joined table
would create another JOIN of the same table.  (As t1 in the
following example).  It doesn't seem to be the intended
behaviour.  As a consequence the filter (WHERE clause in
the following examples) doesn't work as expected.

Queries before this fix looked like the following,
where t1 and t2 are Agent and RouterL3AgentBinding respectively:

    SELECT t2.aaa, t1_1.bbb, ...
    FROM t1, t2 LEFT OUTER JOIN t1 AS t1_1 ON t1_1.ccc = t2.ddd
    WHERE t1.eee = ...;

After the fix, it would be:

    SELECT t2.aaa, t1.bbb, ...
    FROM t2 JOIN t1 ON t1.ccc = t2.ddd
    WHERE t1.eee = ...;

Reference: http://docs.sqlalchemy.org/en/rel_0_9/orm/loading_relationships.html#contains-eager

Partial-Bug: #1414905
Closes-Bug: #1410841
Change-Id: I2243cdfda5c6fe5ef67f96e3274d5381a6e50e62