Added interfaces parsing

[lspgn]

Added interfaces parsing, using FSM templates at the moment.

Can be tested the following way:

import pprint
from napalm_base import get_network_driver
d = get_network_driver('iosxr_ssh')
j = d('127.0.0.1', 'user', 'pass', optional_args = {'port':22})
j.open()
pprint(j.get_interfaces())
j.close()```

[mirceaulinic]

Taking this as-is for the moment, in the iosxr-ssh branch.