[google|dns] Let non-mocked tests run by setting an env variable.

If $FOG_TEST_GOOGLE_DNS_ZONE is set in the environment, the tests run in
non-mocked mode too. Also perform some basic validation for the convenience of
the user - Google API just says "Invalid value for
'entity.managedZone.dnsName'" without hinting on what is wrong.

@icco please review
This is for #2865