Save RTSRoot to a member variable

Get the RTSRoot object on init, and then use it when needed in other
methods.

Signed-off-by: Andy Grover <agrover@redhat.com>
Use new rtslib restore_from_file and save_to_file APIs

As a result it's now better for saveconfig to save/cull backups before
saving the current state, hopefully not a big deal.

Resolves #34

Signed-off-by: Andy Grover <agrover@redhat.com>