Refactors the provider class lookup and kwarg processing so that it can be
reused for plan_output.

dump does not write invalid value(s) to yaml

```
---
'':
    octodns:
        ignored: True
        lenient: True
    type: CNAME # not valid to have a root cname
    value: foo.com.

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.

`Included` and `Excluded` can be used to filter records for one or more specific provider(s). This can be extremely useful when certain record types are not supported by a provider and you want only that provider to receive an alternative record.

See also: https://github.com/github/octodns/issues/26

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