Show ssh username on password prompt

[qris]

When pairing or when working on a machine which isn't on LDAP, the local username is often different to the remote one. The DYE password prompt doesn't make that clear.

[fen-vz-360giving-stage.fen.aptivate.org] Executing task 'deploy'
[fen-vz-360giving-stage.fen.aptivate.org] Login password:

Which "login" is the login password for? It could also be useful to prompt people to login with a different username if necessary. For example this might work:

[fen-vz-360giving-stage.fen.aptivate.org] Logging in as "andybaker". Add "-l <username>" to the fab.py command to change it.