Changing MON Pebble Service command to enter the startup directory first

In rockcraft.yaml WORKDIR is not supported, so it needs to be specified in Pebble service definition by using working-dir parameter.
However, working-dir is not supported in Juju 2.9.x. So, we are entering to the startup directory first as a workaround.

Change-Id: I5949b16ce7e28dfb3f235eabd64062a5a185aa9c
Signed-off-by: Dario Faccin <dario.faccin@canonical.com>