Add from_config() method to EngineFacade

Add a helper that allows to create an EngineFacade instance given an
oslo.config instance. This helper is completely optional, but it was
requested by a few people syncing the latest oslo.db code to target
projects.

Note, that oslo.config is mocked in tests, so oslo.db still has no
runtime dependency on oslo.config.

Blueprint: oslo-db-lib

Change-Id: Icab476386b859be0b0e984a94cbfcf3d581ab6d2
Fixes Guru Meditation portability issue

Nova fails to start on Windows due to non portable signal handling
(SIGUSR1) employed in this patch.

This patch enables the reporting feature only if signal is available
on the current platform.

Change-Id: I579408dddfa18317f4a31eb7d9726bc641eaec04
Closes-Bug: #1286528
Merge "Add from_config() method to EngineFacade"