Deltacloud - record openstack driver realms fixtures for white box unit tests

related to DTACLOUD-477

fix broken build @ https://travis-ci.org/mifo/deltacloud/builds/5039461
Adds 'valid_credentials?' method for digital_ocean driver - DTACLOUD-480

https://issues.apache.org/jira/browse/DTACLOUD-480
Deltacloud - rework 'valid_credentials' - moved to base_driver where possible

Drivers can override this where needed - like e.g. in ec2 driver where just
creating a handle to the aws service (via the appoxy/aws rubygem) isn't enough
to determine that credentials are not valid.