adds bifrost ssh key generation

- This change extend the genpwd.py
  command to generate an ssh key pair
  bifrost.
- This change bifrost config and bootstrap
  task to install the generated keys.
- This change updates the bifrost guide to
  discribe how to provide your own key.

Change-Id: I05243f58843d9195cace253dff5628fae89c78e8
Implements: blueprint bifrost-support