Improve robustness of helm-override-update

- Allow the use of --reuse-values option in helm-override-update
  when there is no user overrides to be consistent with Helm.
- Provide suggestion for correct usage of --set option.

Tests done:
The following commands were used to verify the changes:

system helm-override-update neutron openstack \
--reuse-values --values /home/wrsroot/vlans.yaml

system helm-override-delete neutron openstack

system helm-override-update neutron openstack \
--set conf.plugins.ml2_conf.ml2_type_vlan.network_vlan_ranges=\
"physnet0:10:11,physnet0:1050:1099,physnet1:1100:1124"

Story: 2004520
Task: 29035

Change-Id: Ic5d45f8e7e5e5df4b88468be2f7126c681a6e9cd
Signed-off-by: Tee Ngo <Tee.Ngo@windriver.com>

sysinv/sysinv/sysinv/sysinv/api/controllers/v1/helm_charts.py

@@ -120,13 +120,22 @@ class HelmChartsController(rest.RestController):
                 raise
 
         if flag == 'reuse':
-            file_overrides.insert(0, db_chart.user_overrides)
+            if db_chart.user_overrides is not None:
+                file_overrides.insert(0, db_chart.user_overrides)
         elif flag == 'reset':
             pass
         else:
             raise wsme.exc.ClientSideError(_("Invalid flag: %s must be either "
                                              "'reuse' or 'reset'.") % flag)
 
+        if set_overrides:
+            for overrides in set_overrides:
+                if ',' in overrides:
+                    raise wsme.exc.ClientSideError(
+                        _("Invalid input: One (or more) set overrides contains "
+                          "multiple values. Consider using --values option "
+                          "instead."))
+
         user_overrides = pecan.request.rpcapi.merge_overrides(
             pecan.request.context, file_overrides=file_overrides,
             set_overrides=set_overrides)