vagrant bash completion doesn't ignore plugin commands anymore

When a plugin adds subcommands to vagrant, these would be ignored by vagrants bash completion. This commits fixes the problem.

**Caveat** I have to admit this change slows vagrant bash completion down quite a bit and I have no real idea how to fix this problem. I'm not a bash hacker, though I try to learn.
If vagrant would have a simple command or switch to print out all available commands this would be better, but currently vagrant doesn't even print consistent line indentation for all commands when running `$ vagrant help`. (Some seem to start with spaces, some with tabs.)