kolla designate DNSaaS makes use of containerised bind9 servers
as it's default designate_backend. These can be disabled by
setting designate_backend to "no". default: "bind9"

This commit adds two new properties:
1) designate_backend_external
 which can be enabled by setting it to 'bind9'. default: "no"
and
2) designate_backend_external_bind9_nameservers, which can
 accept a csv list of all the external server addresses.
 (default: "")

The following attributes should either be set:
'internal' (the default)
 designate_backend: "bind9"
 designate_backend_external: "no"
 (designate_backend_external_bind9_nameservers is ignored)
or
 'external'
 designate_backend: "no"
 designate_backend_external: "bind9"
 (designate_backend_external_bind9_nameservers must be populated)

Configuration override files to align with external bind9
dns servers must be supplied manually,
 /etc/kolla/config/designate/rndc.key
 /etc/kolla/config/designate/rndc.conf

Change-Id: I8dbe6fd4fe7820b9143604d89e8399b07e07c3fd