set api.extensions logging to ERROR in unit tests

neutron.api.extensions generates a lot of output in a normal
unit test run. It's responsible for > 80% of the lines in the
subunit_log.txt. Logging each file load 10k times slows everything
down, takes up valuable memory, and doesn't really help with testing.

Instead we can set this to ERROR level in a fixture to ignore most
of that logging and help make the unit tests run faster.

Change-Id: I86b41f20342b4080859ee036220ddffa200f0562
(cherry picked from commit df96a03e6c74f1fceb4525900bca885307907b79)