Improve setuptools capabilities

Make PowerDnsBaseProvider's timeout configurable

fix error on version

adding minimal version only for contributors

define 3 kinds of requirements (base, dev, test)
retrieve version from __init__.py
define setuptools minimal version

in CI install full (base, dev, test) dependencies

OVH: Add support of DKIM records

Add support for included and excluded records

Improve Validations for None/Empty values, Ignore None/Empty values during dump

dump does not write invalid value(s) to yaml

Allow enabling lenient on a per-record basis with octodns.lenient

set default value for nsone cname to None, use first value if non-zer…

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

0.8.8 version bump and changelog

use . instead of source [SC2039]

Add proposed google cloud provider for #23

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

Handle Manager.dump with an empty Zone

add ability to configure update/delete thresholds

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.

This reverts commit 5a3d844559.

Proposed google cloud provider for #23