adminport can be mounted on configurable path-prefix.

Change-Id: Ic9d53b251e924bbed8c0e2b6334103912c4888a1
Reviewed-on: http://review.couchbase.org/37994
Reviewed-by: John Liang <johliang00@gmail.com>
Reviewed-by: Pratap Chakravarthy <prataprc@couchbase.com>
Tested-by: Pratap Chakravarthy <prataprc@couchbase.com>