Disallow a duplicate detach of subnet from router

When a testcase calls add_vn_to_router() two times, as part of addCleanup(),
even the detach of subnet from vrouter is attempted two times
Now, during a second call to add_vn_to_router(), add option to skip cleanup
Update to new module-name DnsAgent in uve queries
Merge pull request #183 from vedujoshi/master

Disallow a duplicate detach of subnet from router