Handling system partition in partition service

This MR introduces logic to handle system partition for azure.

Proposed changes:

1. Users should be able to create/ update system partition using POST/ PATCH methods.
2. Details for system partition should be retrievable using get method.
3. system partition would not be listed via list() call.
4. system partition cannot be deleted using delete call.

Change details:

Added logic to handle aforementioned cases. Added UTs

cc: @polavishnu / @krveduru

PS: The system partition is the new way of handling shared resources in azure. This is not a full-fledged tenant, but a mere encapsulation of system/ shared resources.