OPENDJ-2941 Fix tools extension informations

The issue was a side effect of the configuration framework migration
(see OPENDJ-1238).
With the legacy configuration framework, we ensured that the file
admin/extension.manifest was present in the extension jar like snmp-
mib2605.jar.
Now it seems that we check if the file META-
INF/services/org.forgerock.opendj.config.AbstractManagedObjectDefinition
exists in the extension jar.
In order to prevent unit-tests classpath corruption (with multiple
org.forgerock.opendj.config.AbstractManagedObjectDefinition files) we
now build the snmp-mib2605.jar with an assembly.
The only difference is that there is no more META-INF/INDEX.LIST file in
the new jar.