Fix a usage error of joinedload + filter in dhcp scheduler

Fix a usage error of joinedload + filter in dhcp scheduler

This commit fixes filtering in get_dhcp_agents_hosting_networks.

Also, separate the argument "active" into two; active (heartbeat thing)
and admin_state_up.  Because all in-tree callers with active=True seem
to mean only the former, currently the new admin_state_up argument is not
used.  (Thus this commit doesn't change any behaviour yet.  The argument
might be useful for other changes like [1])

[1] https://review.openstack.org/#/c/147032/

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 NetworkDhcpAgentBinding 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

Closes-Bug: #1414905
Change-Id: Idf9ffdb849de58fba8c18c357ffa10cf0f398140