Adds a new field Plan.exists to indicate whether the zone exists

Implement populate exists for OvhProvider

* spelling: ancillary
* spelling: antarctica
* spelling: australia
* spelling: authentication
* spelling: continental
* spelling: constructor
* spelling: conversion
* spelling: creation
* spelling: doesn't
* spelling: easily
* spelling: efficiently
* spelling: equivalent
* spelling: essentially
* spelling: everything
* spelling: exactly
* spelling: be
* spelling: expensive
* spelling: supports
* spelling: healthcheck
* spelling: immediately
* spelling: ignored
* spelling: invocation
* spelling: itself
* spelling: leftovers
* spelling: missing
* spelling: natural
* spelling: nonexistent
* spelling: peculiarities
* spelling: pointing

This change hit a line length limitation, so I'm wrapping it and
adding a period which appears to match local style...

* spelling: quicker
* spelling: response
* spelling: requested
* spelling: redirect
* spelling: traffic
* spelling: unknown
* spelling: uploaded
* spelling: useful
* spelling: separately
* spelling: zone

Add DKIM to provider capabilities

Change str() to unicode() to avoid encoding problems

Fix handling of Cloudflare ALIAS updates

Extract _record_for from populate, use round trip in _apply_Update

fix logging for update/delete_pcent_threshold

This will ensure that we have exactly the same logic/behavior across the
board when turning records into content and prevent the :-( hack that was in
here before. It was missing the max(min, ttl) bit we throw everything else
through and this makes that consistent. Most importantly it'll prevent us
from having to fix bugs or make improvements in multiple code paths in the
future.

Remove unnecessary argument "cdn" from CloudflareProvider

Properly set filter rules for geo answers

Generate the zone name following the spec of Google Cloud DNS

Zone name must begin with a letter, end with a letter or digit, and only contain lowercase letters, digits or dashes, and be 63 characters or less.
For instance, a reverse zone of IPv6 may violate the spec on the first character and the length of the zone name.

Support for SSHFP ECDSA (3) and SHA-256 (2)

Fix SRV and SSHFP record for OVH provider

RFC 6594 & RFC 7479

Option to handle Cloudflare proxied records

Add SRV support to Cloudflare provider

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

Dnsmadeeasy provider