vstart_runner: maintain log level when --debug is passed

vstart_runner: maintain log level when --debug is passed

When --debug and --clear-old-log options are passed to vstart_runner.py,
it ends up resetting log level to the default level (which is
logging.INFO) despite of --debug. Set "log_level" as default parameter
to init_log() so that the log level code for clearing old log can pass
current logging level to init_log() to maintain the log level user
desired for.

Fixes: https://tracker.ceph.com/issues/51344
Signed-off-by: Rishabh Dave <ridave@redhat.com>