Update Traffic Router Watchers to use a parameter for API version

[mattjackson220]

This Improvement request (usability, performance, tech debt, etc.) affects these Traffic Control components:

• Traffic Ops (maybe)
• Traffic Router

Current behavior:

Currently, the Watchers in Traffic Router (like LetsEncryptDnsChallengeWatcher and FederationsWatcher) have the Traffic Ops API version hard coded.

New behavior:

This should be updated to be a parameter so a new deployment isnt required to change API versions. The new behavior should just use a changeable API version instead of the hardcoded one

[rawlinp]

Rather than just controlling the API version, we should have the parameter control the entire URL. That is how most of the TR watchers work today.

[mitchell852]

this is currently hard-coded to api 2.x, correct?

[mattjackson220]

yea the default url is hardcoded to 2.0

[mitchell852]

this is a nice-to-have imo and can be removed from the 7.0.0 milestone IF the default url is hardcoded to a non-deprecated API version (i.e. 3.0+) and i think @rawlinp was going to do that as part of #6653

[zrhoffman]

We're using a constant for TO API version as of #6778, which seems preferable so an operator does not have to manually change the API version in the configuration each a TR update is intended to change the API version used.