pmdasnmp: add Install checks for the required Net::SNMP module
pmdasnmp: add Install checks for the required Net::SNMP module

Since we're using soft dependencies for this PMDA on some of our
platforms, let the user know as early as possible of problems and
with a clear diagnostic message.

Related to Red Hat BZ #1790433

adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp