Role specifies whose tasks will run on the NodePool. The roles
can be specific to workloads. Exactly one GkeNodePoolTarget within
the VirtualCluster must have 'default' role, which is used to run
all workloads that are not associated with a NodePool.
Values:
ROLE_UNSPECIFIED (0):
Role is unspecified.
DEFAULT (1):
Any roles that are not directly assigned to a NodePool run
on the default role's NodePool.
CONTROLLER (2):
Run controllers and webhooks.
SPARK_DRIVER (3):
Run spark driver.
SPARK_EXECUTOR (4):
Run spark executors.
