Add an option to create DNS 'A' records in Cloud DNS when a node is created

This PR adds an option "--rackspace-add-dns-record" that will create an A record within Cloud DNS when a node is created.  This option takes a DNS zone name as a parameter and create an A record of <node name>.<zone name> that points to the public IP of the new node.   This PR is a continuation of the work in the previous PR I submitted:

https://github.com/opscode/knife-rackspace/pull/42

This new PR depends on the work in PR #42, which adds more intelligent determination of the correct public IP in RackConnect environments.