Drop SCYLLA_EXT_OPTS

[dkropachev]

Currently if we want to pass additional arguments to scylla we use SCYLLA_EXT_OPTS environment variable like so:

    os.environ['SCYLLA_EXT_OPTS'] = "--smp 1"

Beside that it is ugly, It creates conflicts if we would want to run tests in parallel. After ccm issue: https://github.com/scylladb/scylla-ccm/issues/675 is merged, we need to replace SCYLLA_EXT_OPTS with proper parameter.