CLOUDSTACK-6998: GloboDNS, Integration with external DNS Provider

This is a feature to handle DNS entries by means of an external DNS Provider,
such as Bind. These entries include DNS domains and reverse domains, VM records
and reverse records.

For a complete description, please refer to the design document available at
https://cwiki.apache.org/confluence/display/CLOUDSTACK/Bind+and+PowerDNS+integration+by+Globo+DNSAPI

For the discussion about this feature on the dev mailing list, please refer to
http://markmail.org/thread/fvwf36hpxotiibka

Summary:
- new Network Service Provider called GloboDNS
- new Network Element to manage network domains and VM records (entries) on an external API
- new Network Resource to communicate with GloboDNS (open source)
- new API command to add DNS server
- new global option to determine if this provider should override VM entries on external DNS server
- changes in UI to include GloboDNS in Providers list

Signed-off-by: Rohit Yadav <rohit.yadav@shapeblue.com>
apidoc: add GloboDns entry under network section

Signed-off-by: Rohit Yadav <rohit.yadav@shapeblue.com>