add 'known_host' module

This module manages SSH known_host entries in a known_hosts file.
It does this using ssh-keyscan and ssh-keygen.
digital_ocean.py enhancements

* Always refresh droplet information (versus getting it from cache) for
  --list, --host, --droplets, and --all.   All the DigitalOcean calls are
  pretty fast and without this change, inventory was often stale/incorrect.
  For example, previously if you destroyed a droplet, the inventory
  script would still find it and attempt to act on it.

* Added --force-cache option

* Added some option shortcuts -d, -r

* Added more value sanitization
digital_ocean: make sure 'images' includes non-global images

The `dopy` library function all_images includes a 'global' filter
by default.  This was preventing private images from showing up
in --images.

The problem also made this script fail immediately for people
who had droplets previously created with private images.
Merge branch 'digital_ocean_inv' into devel
Merge branch 'known_hosts' into devel
digital_ocean: whitespace changes
Merge branch 'digital_ocean_inv' into devel