qa/workunits/rados: skip running envlibrados rocksdb tests on ubuntu

qa/workunits/rados: skip running envlibrados rocksdb tests on ubuntu

This test passes on centos and rhel, but fails on ubuntu from an
invalid pointer. Since the envlibrados rocksdb tests are experimental
and don't have any actual users, we can just run them on rhel and
centos.

At the moment, the actual bug is not fully understood, but it was
decided that fixing it is low priority, and removing the test from
problematic distros is okay for the time being. This commit
is considered a workaround to the actual issue.

Related tracker: https://tracker.ceph.com/issues/57632
Signed-off-by: Laura Flores <lflores@redhat.com>