Rework how we gather model number from targets

[ventris]

Currently we have a list of oids that we scrape to get the model for the model specific oids, and the first one that got data will be the one used.

snmpexporter/snmpexporter/snmpimpl.py

Lines 131 to 137 in 2315184

	def model(self, target):
	model_oids = [
	'.1.3.6.1.2.1.47.1.1.1.1.13.1', # Normal switches
	'.1.3.6.1.2.1.47.1.1.1.1.13.1001', # Stacked switches
	'.1.3.6.1.2.1.47.1.1.1.1.13.10', # Nexus
	'.1.3.6.1.2.1.1.1.0', # Other appliances (sysDescr)
	]

This is a problem with juniper mx10k routers as they will send its id number on 1.3.6.1.2.1.47.1.1.1.1.13.1 and not the readable model number.
The model number is send by the 1.3.6.1.2.1.1.1.0 oid on those routers and that is last in the current list.

Evaluate if it is possible to change this without breaking all other targets.

[Zmegolaz]

I suspect this is the case for more Juniper models, since there's already an entry for ^710-.*.