Changing NBI 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: Ia441b5445822f0833f659f964fc1f2216586cdc5
Signed-off-by: gatici <gulsum.atici@canonical.com>
diff --git a/installers/charm/osm-nbi/config.yaml b/installers/charm/osm-nbi/config.yaml
index 961cfff..bed55da 100644
--- a/installers/charm/osm-nbi/config.yaml
+++ b/installers/charm/osm-nbi/config.yaml
@@ -60,9 +60,7 @@
     default: 20
     description: Max allowed body-size (for file uploads) in megabytes, set to 0 to
       disable limits.
-    source: default
     type: int
-    value: 20
   tls-secret-name:
     description: TLS secret name to use for ingress.
     type: string
diff --git a/installers/charm/osm-nbi/src/charm.py b/installers/charm/osm-nbi/src/charm.py
index 00ef425..4a0bbf1 100755
--- a/installers/charm/osm-nbi/src/charm.py
+++ b/installers/charm/osm-nbi/src/charm.py
@@ -267,10 +267,11 @@
                 "nbi": {
                     "override": "replace",
                     "summary": "nbi service",
-                    "command": "python3 -m osm_nbi.nbi",
+                    "command": "/bin/sh -c 'cd /app/osm_nbi && python3 -m osm_nbi.nbi'",  # cd /app/osm_nbi is needed until we upgrade Juju to 3.x
                     "startup": "enabled",
                     "user": "appuser",
                     "group": "appuser",
+                    "working-dir": "/app/osm_nbi",  # This parameter has no effect in juju 2.9.x
                     "environment": {
                         # General configuration
                         "OSMNBI_SERVER_ENABLE_TEST": False,