Changing RO 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: I44bbe0beeafce3932218a0df1d38191a14cb4a8a
Signed-off-by: gatici <gulsum.atici@canonical.com>
diff --git a/installers/charm/osm-ro/src/charm.py b/installers/charm/osm-ro/src/charm.py
index 84c0ee3..89da4f1 100755
--- a/installers/charm/osm-ro/src/charm.py
+++ b/installers/charm/osm-ro/src/charm.py
@@ -302,10 +302,11 @@
                 "ro": {
                     "override": "replace",
                     "summary": "ro service",
-                    "command": "python3 -u -m osm_ng_ro.ro_main",
+                    "command": "/bin/sh -c 'cd /app/osm_ro && python3 -u -m osm_ng_ro.ro_main'",  # cd /app/osm_nbi is needed until we upgrade Juju to 3.x.
                     "startup": "enabled",
                     "user": USER,
                     "group": GROUP,
+                    "working-dir": "/app/osm_ro",  # This parameter has no effect in Juju 2.9.x.
                     "environment": {
                         # General configuration
                         "OSMRO_LOG_LEVEL": self.config["log-level"].upper(),