This commit adds support for zones aliases. This allows to define one or
multiple zone as aliases of an existing zone without using workarounds
like simlinks and miltiple "zones" entries in the configuration file.

An alias zone is share all of its content with it parent zone, only the
name of the zone is different.

```
zones:
  example.com.:
    aliases:
      - example.net.
      - example.org.
    sources:
      - in
    targets:
      - out
```

Known issues:
- No documentation,
- Only the `octodns-sync` and `octodns-validate` commands supports
aliases zones at this time,

I added a loop in the manager init function which convert all alias
zone to "real" ones during config validation, however I'm not sure
this is the right approach. Comments welcome.

* Removed conversion of DYN records to A records
* Updated test fixtures to change test DYN to an A record

* Removed conversion of URL and STEALTH records to CNAME records
* Updated test fixtures to remove URL conversion testing

* Replaced camelCase with snake_case
* Change apikey to api_key
* Added check on record name before delete during domain_create

Per the discussion on https://github.com/github/octodns/issues/583
here is a work in progress of environment variable injection for
discussion.

Root NS modification support is being developed on
https://github.com/github/octodns/pull/434

Removing attempts to validate this functionality for now.

This provider class for easydns.com adds support for basic dns records
through the easyDNS v3 API.  Support for dynamic and geo based dns
records is planned for a future update.

Sample configuration for the easyDNS provider are:

  easydns:
     class: octodns.provider.easydns.EasyDNSProvider
     token: <token>
     apikey: <key>

The token and key values are found on the easyDNS customer portal at:

https://cp.easydns.com/manage/security/api/production_info.php

Also, below are some optional configuration parameters which can be
added to override the class defaults.  By default the provider class
connects with the LIVE easyDNS API, if you wish to perform testing
with the easyDNS Sandbox API you can enable it by adding the following
configuration parameter:

     sandbox: True

Note, the API token and key are different for the sandbox than they
are for the production API, you can obtain sandbox credentials at:

https://cp.easydns.com/manage/security/api/sandbox_info.php

Lastly, if you have created Domain Portfolios through the easyDNS CP
you can configure which portfolio new domains will be added to by
supplying the portfolio option with the name of your portfolio.

     portfolio: <portfolio name>

- For providers that support such

* Add a client_config option to Ns1Provider, for passing additional
  options or overrides to the SDK config. This should allow NS1 users
  some flexibility without bothering octodns so much.
* Expose the actual SDK client object as `_client` on the Ns1Client wrapper
* Do my best to clarify options and defaults in the Ns1Provider docstring

Adds a "parallelism" argument to the NS1 Provider. If set, we analyze
response headers and attempt to avoid 429 responses.