Phase2

Most of this has been in alpha use over the weekend. New additions *not* in alpha production include:
1. Documentation
2. PxeManager __init__ has 2 more parameters that allow for specifying the username/password to use for ssh connection

The initial ssh stuff is serial. I don't think this is critical right now to move this to threads. What happens is the ssh connection tries and tries until it succeeds. then move to the next. After the first, the other hosts *should* be up and each will have successful ssh connection in 1 possibly 2 attempts. So the time will not greatly increase with a threaded model. However, IMHO, it is the right thing to do later.
@tbeckham what do you think of renaming these ssh_user and ssh_password?