specify buildslave subcommand function in it's Options class

Use subcommandFunction class variable of the subcommand's Option flag
to specify the implementing function. This is the same style used in
master cli commands implementation.
test handling of invalid arguments to 'buildslave' cli command

Added unit tests on code in buildslave.scripts.runner.run() that
handles invalid command line arguments.
unit test successful invocation of 'buildslave' cli command
unit test options parsing for 'buildslave' command without arguments

Test that buildslave.scripts.runner.Options class flags invocation of
'buildslave' command without arguments, as invalid.
unit test handling of '--verbose' option for 'buildslave' command
unit test handling of '--version' option for 'buildslave' command
use StdoutAssertionsMixin to check prints to stdout
unit test buildslave.scripts.runner.MakerBase class
unit test buildslave.scripts.runner.UpgradeSlaveOptions class
unit test buildslave.scripts.runner.RestartOptions class
unit test buildslave.scripts.runner.StopOptions class
unit test buildslave.scripts.runner.StartOptions class