SERVER-14419 Correctly parse the "masterHost" argument to Cloner::go into a ConnectionString, not a HostAndPort.

The argument represents a ConnectionString, but we parse it into a HostAndPort
when doing isSelf checks.  This was an error before that led checks to succeed
incorrectly, but fails with the new HostAndPort parsing code from SERVER-14419.
SERVER-14419 Introduce unittest of HostAndPort; make ":123" unparseable due to lack of host component.
SERVER-14419 Implement HostAndPort::parse.