RFC 6594 & RFC 7479

This change imports records that are marked as proxied so that they can be synced to other DNS providers as described in
[this support acticle](https://support.cloudflare.com/hc/en-us/articles/115000830351-How-to-configure-DNS-for-CNAME-partial-setup-when-managing-DNS-externally).
Records that use this functionality will be ignored by this provider and not be synced back to Cloudflare as we don't know the origin record values that would be required.

This change does not allow you to enable, disable or configure the CDN itself as that would require a lot of metadata to be handled by OctoDNS.
The intention of this change is to allow users to run a multi-DNS provider setup without sending any traffic to their origin directly.

See also github/octodns#45

Translates them to/from root CNAME

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.