As part of the transition from Python 2 to Python 3, we are installing both pip2 and pip3 in the slave and config-engine containers. This PR replaces calls to `pip` in these containers with an explicit call to `pip2` to ensure the proper version of pip is executed, no matter which version of pip is aliased to `pip`, as we no longer rely on that alias.
Also some other pip-related cleanup
The -sv2 suffix was used to differentiate SNMP Dockers when we transitioned from "SONiCv1" to "SONiCv2", about four years ago. The old Docker materials were removed long ago; there is no need to keep this suffix. Removing it aligns the name with all the other Dockers.
