Wrong DOM values for xe-* interfaces #178

[Gaste8]

Fixes #178.

For interfaces with zero lanes there is the tag "rx-signal-avg-optical-power" instead of
"laser-rx-optical-power" in the xml/json response and vice-versa.

Added clause to fix this.
-- If no value is available then try the other one.

Additional Question:
some devices use [0.0000 mW / - Inf dBm]
and others use [0.0001 mW / -40.00 dBm] as a lower bound for diagnostics values.
Should we set a standard and change one of these?

[fstolba]

FWIW we have this patch running on our production cluster for 2+ weeks without any problems 👍 .

It will certainly make sense to standardize on a single value to signify No signal. As far as I can tell, the value that will be reported is dependent on the transceiver model in use (probably transceiver vendor too) and JunOS version. For example 10G-LR shows -40 while 10G-SR shows -Inf in the same box.