Add GetStoragePoolVolumes api to get all volumes connected to given s…

[alikdell]

Description

Add GetStoragePoolVolumes api to get all volumes connected to given storagepool

GitHub Issues

List the GitHub issues impacted by this PR:

GitHub Issue #
#dell/csm#1221

Checklist:

• I have performed a self-review of my own code to ensure there are no formatting, vetting, linting, or security issues
• I have verified that new and existing unit tests pass locally with my changes
• I have not allowed coverage numbers to degenerate
• I have maintained at least 90% code coverage
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• Backward compatibility is not broken

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Please also list any relevant details for your test configuration

• # go test -v -run TestGetStoragePoolVolumes
  === RUN TestGetStoragePoolVolumes
  --- PASS: TestGetStoragePoolVolumes (0.26s)
  PASS
  ok github.com/dell/goscaleio/inttests 0.346s

• Run manual test to get list of volumes belong to storagepool. Also used in CSM Authorization