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

This implements it transparently at Record level. Providers that need things to
be chunked (seems to just be Route53 an Dyn) switch to use `chunked_values`, but
everything else can stick with `values`. I've run through each provider I have
access to verifying that things operate as expected/required. OVH and Azure are
untested.

Seem to be pretty inconsistently implemented/validated across providers so
just shrug and move on.

Before, 1-2k record took ~10s and more than that was just painful, 5k took
forever. This records things to keep a dict of nodes with a set of records so
that we can quickly "jump" to the point we're looking for without having to
search. 10k records now takes ~5s.

Support replace=True in zone.add_record

- unrecognized wording for invalid values

- punting on service, regex & replacement validation for now
- clean up MX a smidge

- adds lenient flag to Record.new, problems during validation are just
  warnings if it's true
- target populate calls during the plan phase pass lenient=True
- make all of the provider.populate call logging consistent including both
  target and lenient
- add source=self to Record.new in a few places that were missing it

This is with an eye toward expanding it in the future both in terms of what it
checks and to add the ability to ignore things. This commit does not intend to
change any validation. It only reworks the flow and improves the error
messaging.

Thanks @offmindby!