fixed PEP8 violations
added shell completion hints

For 'buildbot' and 'buildslave' subcommands that take
base directory as first argument, suggest directories.

For 'buildbot checkconfig' suggest files as first
argument.
added zsh completions files

Added files that can be used to enable tab-completion in zsh
for 'buildbot' and 'buildslave' commands.
added bash completions files

Added files that can be used to enable tab-completion in bash
for 'buildbot' and 'buildslave' commands.
monkeypatch shell completions defintions for older twisted

The shell completions support was added in twisted 11.1.0. When
running with older twisted, add completions definitions we are using
to twisted.python.usage module.