Zone name must begin with a letter, end with a letter or digit, and only contain lowercase letters, digits or dashes, and be 63 characters or less.
For instance, a reverse zone of IPv6 may violate the spec on the first character and the length of the zone name.

use uuid4().hex to ensure unique zone_name generation and thereby streamline with the other providers.

Cache googleclouds zones so that populate dont have to list all each time called.

* in _rrset_for_X functions, use values instead of data attribute.
* Small typo fixes and removals of redundant steps etc.
* Unset GOOGLE_APPLICATION_CREDENTIALS in coverage script.

remove _GoogleCloudRecordSetMaker into the GoogleCloudProvider, and consolidate methods.

Also make _data_for_SPF and _data_for_TXT the same method.

Proposed google cloud provider for #23