Add more tests for storageops

[piyush-nimbalkar]

Is this a BUG REPORT or FEATURE REQUEST?:
BUG REPORT

What happened:
Need more robusts tests for storageops

What you expected to happen:
There should be more tests here to test idempotency and difference in behavior for these calls for the different implementations.

• Call Create() for an already existing disk
• Test for all APIs on non-existing disks
• Attach an already attached disk
• Detach an already detached disk
• Try to delete an attached disk
• Verify snapshot is present after the source disk is deleted (some storage might behave differently)

How to reproduce it (as minimally and precisely as possible):

Anything else we need to know?:

Environment:

• Container Orchestrator and version:
• Cloud provider or hardware configuration:
• OS (e.g. from /etc/os-release):
• Kernel (e.g. uname -a):
• Install tools:
• Others: