Add '_has_dns_extension' function
Add '_has_dns_extension' function

This is how we've standardized on checking for extensions. Use it for
DNS too.

Change-Id: I46b24fe630fdfb9ea49c70ffc2d7386c9c31b5f0

mikalstill/nova,openstack/nova,klmitch/nova,openstack/nova,mikalstill/nova,klmitch/nova,mikalstill/nova,mahak/nova,rahulunair/nova,klmitch/nova,mahak/nova,gooddata/openstack-nova,phenoxim/nova,rahulunair/nova,mahak/nova,gooddata/openstack-nova,gooddata/openstack-nova,phenoxim/nova,openstack/nova,rahulunair/nova,klmitch/nova,gooddata/openstack-nova