This commit loads all datasources drivers by default without the need
of config option. The config option 'drivers' would be deprecated as it
is required to update manually on newly supported drivers for each release
and also error prone.
To disable any of the drivers, can be disabled using the config option
'disabled_drivers'
For example,
disabled_drivers = plexxi, murano
TODO: Support disabled_drivers option to disable the datasource drivers
Partially-Implements blueprint enable-drivers-by-default
Change-Id: I5f9878d07c2e1487f8f14f7cd1948560327d8083
82a708abcf