Add checks for master IP in cluster verify

This also updates a comment in the unittest for utils.py. We unittest
the new function for two things: correct reporting on real case (for
localhost), and correct reporting with a mocked-out TcpPing that returns
false.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>