Fix typos for attributes of constants

This patchs fixes the typo in
neutron/services/dynamic_routing/dr_rpc_base.py.
constants.DR_AGENT_SCHEDULER was replaced by
constants.DR_AGENT_SCHEDULER_EXT_ALIAS.

Change-Id: I644c4278acc328cab12733a74e93e0fb97ee4721
Signed-off-by: Taku Fukushima <tfukushima@midokura.com>