Vendor go-homedir

Signed-off-by: Ben Firshman <ben@firshman.co.uk>
Add basic host management with drivers
Add Azure driver for hosts
Only create HTTP transport when required

The current Docker client assumes that there is always a URL
available, but this might not be the case if the host is stopped.
Add maintainers file to hosts

Signed-off-by: Ben Firshman <ben@firshman.co.uk>
Display errors for drivers that can't do stuff
Only show SSH key generation output in debug mode
Add "error" state for hosts
Throw error when DO IP address doesn't exist
Add stubs for CLI documentation
Add force removal of hosts

... and clean up local configuration when create fails