feature(nemesis): introduce MemoryStressMonkey
feature(nemesis): introduce MemoryStressMonkey

The new nemesis tries to execute stress-ng to preemt scylla memory,
scylla lock memory is enabled by default, so scylla won't swap allocate
memroy.

Signed-off-by: Amos Kong <eb01d8f828a6c9a20be4534e72e049aaf41503df@scylladb.com>

scylladb/scylla-longevity-tests,scylladb/scylla-longevity-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-longevity-tests,scylladb/scylla-cluster-tests