core: accept passwords in ssh_info
communicators/ssh: support passwords
core: warn if password only on `vagrant ssh`
core: raise exceptions if they happen in WaitForCommunicator threads
insert_public_key cap, and use that if it exists
core: fix potential exception case in SSHExec middleware
communicators/ssh: output UI when inserting key
core: error if vagrant ssh -c with password
communicators/ssh: if insert_key is false, don't insert a key
website/docs: update docs for insert_key
Merge pull request #2754 from mitchellh/f-insert-private-key

Support password-based SSH, automatically insert insecure key