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 -> #266

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This change is 

[coveralls]

Coverage increased (+0.2%) to 57.988% when pulling acb92fa on fix/ub-1671_idempotent_delete into ae6a0a5 on dev.

[shay-berman]

NOTE: there is one comment to fix, please check it. And if it passed staging then feel free to merge to dev.

Reviewed 8 of 8 files at r1.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @olgashtivelman)

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local/scbe/datamodel_wrapper.go, line 103 at r1 (raw file):

		// sanity
		if d.dbVolume == nil {
			return d.logger.ErrorRet(&resources.VolumeNotFoundError{VolName: name}, "failed")

I think its better to add here a logging for idempotent, like we do in many other cases of idempotent.
Of course not to raise error just message to the log that this is the case.

[olgashtivelman]

Reviewable status: 7 of 8 files reviewed, 1 unresolved discussion (waiting on @shay-berman)

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local/scbe/datamodel_wrapper.go, line 103 at r1 (raw file):

Previously, shay-berman wrote…

I think its better to add here a logging for idempotent, like we do in many other cases of idempotent.
Of course not to raise error just message to the log that this is the case.

Done.