break out a bunch of common functions in monkeysphere-host:

break out a bunch of common functions in monkeysphere-host:
- create_*_*_file to create the key files
- load_*fingerprint to load the host fingerprint into an exported
  variable (HOST_FINGERPRINT)
- check_host_*key to check for the presence of a host key
modified {import,gen}_key to use these new functions.