Fix/ub 1671 idempotent issues in delete

[olgashtivelman]

in this PR the idempotent issues in the delete request are fixed. the issues are as follows: (in delete volume function)

• in get Back-end in the storage_api_handler delete function - if volume does not exist then return nil.
• in get volume: if volume does not exist then return success
• in vol delete from XIV : if an 404 error is returned from SC then continue with the flow. (to delete from ubiquity DB)
• in delete volume from DB (for the case where we have a non-db volume) if a volume not found error is returned then return success. in all the above issues an idempotent there is a warning message in the logger.

comment : an assumption is made that if a volume is removed from the DB then it is definitely removed from the storage since the removal from the DB is the latter operation.

Note: addition PR to handle delete idempotent in the flex side -> https://github.com/IBM/ubiquity/pull/266

---

This change is 

[coveralls]

Coverage increased (+0.2%) to 57.988% when pulling acb92fa237e2af4a1d82d662e8771a19534370f6 on fix/ub-1671_idempotent_delete into ae6a0a55dd776cfac7fa96c973b9f7691e2c6e2a on dev.