tools: frr-reload.py for single daemon

tools: frr-reload.py for single daemon

allow frr-reload.py to be invoked with a --daemon option to specify
an individual daemon for which the configuration diff should be
computed. This is useful when integrated config is not used and we
want to apply a patch to a single daemon config file.

No attempt to integrate this with 'service frr reload' has been done.
Making watchfrr work with per-daemon config is outside the scope of
this simple patch.

Signed-off-by: Emanuele Di Pascale <emanuele@voltanet.io>