Fix scripts so they no longer use Netbox.snmp_version

[hmpf]

Scripts that still access Netbox.snmp_version triggers the warning

/usr/bin/navSCRIPT:73: DeprecationWarning: The Netbox.snmp_version attribute will be removed in the next feature release

This can make it difficult to eyeball what the script actually returns and needs to be updated to the new method before snmp_version disappears as an attribute.

[johannaengland]

Notes from an irl discussion with @lunkwill42
This issue basically consists of three steps:

1. Create a new public method on Netbox that returns a preferred management profile. This function basically works the same way that _get_snmp_config currently works: return the management profile with the highest snmp version which is writable or not (or both) depending on a given parameter.
2. Create two more public methods on Netbox that call this method with the writable parameter either being True or False so to get either a read-only or a read-write management profile.
3. All previous usage of the .snmp_version function should be changed to call one of the methods from step 2.

[lunkwill42]

This issue does not cover ipdevpoll. Ipdevpoll needs its own issue.