Validate 'master' option for buildbot cli subcommands.

Make sure that 'master' option for buildbot try, debugclient, statuslog, statusgui and sendchange cli subcommands are validated. Currently only 'buildbot user' command checks that master option is in 'hostname:port' format.

With this patches, all the above commands will print a user friendly error message, instead of exception Traceback, when splitting/parsing the master string fails.

I also added some unit tests on 'buildbot user' options validation code and removed dead code on this branch.