add uninstall, purge

Mirror the platform detection for install.  Obviously this needs cleanup
to support other platforms.

Signed-off-by: Sage Weil <sage@inktank.com>
new: require monitors

It doesn't make much sense to run this command without specifying the
mons, unless you are going to edit the file manually.  If that is the
case, however, we can still require it and they can change the names or
IPs after.

Signed-off-by: Sage Weil <sage@inktank.com>