Make the SNMP agent optional when OPENJDK=true and IMPORT_BINARY_PLUGS=false
in order to avoid build breaks if the closed SNMP files are not availaible.

At runtime, the SNMP agent is activated only if -Dcom.sun.management.snmp.port=<port> is
present. The goal of this change is to allow to build a JDK that doesn't contain the
SNMP agent, and throws an UnsupportedOperationException if the
-Dcom.sun.management.snmp.port=<port> is present.