[google] Implement disk mocks and enable tests

Avoid repeating api_version
Add persistent disk to insert_server mock
Use Fog::Mock.delay properly instead of Fog.timeout
Update default image
[google] Alias flavor_id and machine_type for consistency with other providers
return NS and SOA records as per https://github.com/fog/fog/issues/2419 - this allows us to work with NS subdelegations, as well as the NS and SOA records while the type can't be changed, the values and TTLs can be
[AWS|Storage] fixed signed urls when using session tokens

the session token needs to be added to the signature as a header
(as well as being included in the query string)
fix for broken AWS records tests

see #2439
Update excon dependency to version ~>0.30.0