Support filtering of AWS zones to manage (to resolve multiple zones with same domain)

[joshsouza]

Similar to #335, we have multiple zones defined in our AWS configuration for the same domain name. This provides the ability for multiple VPC's to resolve their own versions of services (I.E. you can always resolve ldap.mydomain.com but the answer might differ depending on what VPC you are in). Presently kops/dns-controller has a --zone flag that allows you to restrict which zones are being managed, which is how we currently operate (each k8s environment's DNS controller can only manage the VPC-specific zone it belongs to). However this could also be used to specify whether you want to update the public/private zones etc... so if external-dns were to similarly add a filter for what zones to manage in aws it could potentially resolve #400, #335, #322, and #287 (and I'm sure others). I also think that coupled with an adjustable hostnameAnnotationKey you could run multiple external-dns instances to manage your public/private records by using different annotations.

Proposed idea:
Add a --aws-zone-filter as mentioned in #287, which restricts what AWS zones are permitted to be managed by external-dns, and what zones will be queried.

Expected behavior:
In an environment with multiple VPC's, and multiple private Route53 zones for example.com you can restrict external-dns to only manage the desired zone based on a parameter.