StoragePools from storage drivers of scope local do not get cleaned up when hosts are removed

[cloudnautique]

Rancher Versions:
Server: v1.5.x

Environment Type: (Cattle/Kubernetes/Swarm/Mesos)
Cattle
Steps to Reproduce:
Create a cattle environment, and deploy the secrets volume driver.
Note on the Infrastructure->Storage tab the Storage pools.
Remove a host, and observe that the storage pool still exists on the storage tab.

Results:
Storage pools that are scoped local to the host, should be removed when the host is removed from the system.

[sangeethah]

Tested with v1.6-development branch.

Create a cattle environment with 2 hosts .
Deploy the rancher-secrets and secrets-bridge-v2 volume driver.
Remove a host.
Storage pool does not list the removed host anymore.