[GONRG-6177]Add externals sets variable
- Added separate variable $GC_HELM_SETS to pass service specific --sets from service level;
- Removed nowhere used $ISTIO_ENABLED variable;
- Removed $GC_PARTITION_NAMESPACE variable as Partition Service specific only. Value now is passed via $GC_HELM_SETS from Partition Service.
Edited by Yurii Pelykh [EPAM / GCP]