Fix no attribute 'is_dvr_router' error in test_l3_agent_scheduler

cls.is_dvr_router attribute is invalid in the case which result of the
is_extension_enabled() called in resource_setup() is false.
This patch fixes it.

Closes-bug: #1465623
Change-Id: If3e9f80991b2675b4700c7cdc80b5689b444d40b
1 file changed