Add missing explicit cfg option import

The test ceilometer.tests.event.test_endpoint.TestEventEndpoint cannot
be run standalone because of missing import.

This change fixes that.

Change-Id: I18083a0793204da5ef9b78573fa4a0483964e158
Fix notification for NotImplemented record_events

With mongodb as storage backend, notification-to-sample will be broken by
NotImplementedError of record_events() method if we have set
"store_events" option as True in ceilometer.conf.

This patch catch the NotImplementedError and enable the
notification-to-sample process.

Change-Id: I7ace5a8581518fa56e58abca2e377b513ff38675
Closes-Bug: #1320420
Refactor tests to remove direct access to test DBManagers

Previously in several tests DBManager was specified explicitly and some
tests tried to use it even if no backend was available.
Now we should not access them directly. Instead we can mark certain
tests as applicable only to certain backend driver.
One will only have to provide an URL for tests to use.

Later this code will be used to integrate jenkins jobs on real backends.
Database URL will be provided by jenkins and tests will figure out which
manager should be used and what tests can run with it.

Related to blueprint sql-unit-tests-on-real-backend

Change-Id: I9910c25fffb971addec9cec33b624e3b8387f310
Merge "Fix notification for NotImplemented record_events"
Merge "Add missing explicit cfg option import"
Merge "Refactor tests to remove direct access to test DBManagers"