Hardening for location awareness command-line flag

Add few verification steps related to the location assignment:

• the location assignment executable is present and executable
• the location assignment executable conforms with the expected interface: it accepts one argument (IP address or DNS name) and outputs the assigned location into the stdout
• the same DNS name/IP address assigned the same location
• the result location output into the stdout conforms with the format for locations in Kudu

It's possible to implement these in kudu-master using group flag validators: see the GROUP_FLAG_VALIDATOR macro.

Performing few verification steps mentioned above should help to avoid situations when Kudu tablet servers cannot be registered with Kudu master if the location assignment executable path is misspelled or the executable behaves not as expected.