Allow explicit, start-of-run sudo password entry

[bitprophet]

Feature

Add an option triggering the sudo/login password prompt before any tasks actually execute. This fills in the default password cache for the rest of the run.

Rationale

• Frequently, one needs to run sudo commands in parallel & needs password entry (passwordless sudo not always an option).
• Because of the parallelism, prompting at runtime is not possible (Fab will blow up if the remote end indicates it needs to prompt).
• Priming the password cache via --password or env.password is insecure and/or clunky.