Unify the oslo_log import usage

In watcher project, some py file use "from oslo_log import log as logging"
for import, but more use "from oslo_log import log"
like https://github.com/openstack/watcher/blob/master/watcher/decision_engine/strategy/strategies/dummy_strategy.py#L19
this patch to unify it.

Change-Id: I9c667bb202bebc7942d1af5ce438375285c2e2ba

watcher/common/context.py

@@ -11,11 +11,11 @@
 # under the License.
 
 from oslo_context import context
-from oslo_log import log as logging
+from oslo_log import log
 from oslo_utils import timeutils
 import six
 
-LOG = logging.getLogger(__name__)
+LOG = log.getLogger(__name__)
 
 
 class RequestContext(context.RequestContext):