update "Volume Delete" interface

[simrannagra]

What this PR does / why we need it:This PR updates Volume delete interface

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #

Special notes for your reviewer:

Release note:

[coveralls]

Pull Request Test Coverage Report for Build 214

• 11 of 14 (78.57%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.002%) to 69.713%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
openstack/blockstorage/v2/volumes/requests.go	11	14	78.57%

Totals
Change from base Build 213:	0.002%
Covered Lines:	11253
Relevant Lines:	16142

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 244

• 11 of 14 (78.57%) changed or added relevant lines in 1 file are covered.
• 203 unchanged lines in 5 files lost coverage.
• Overall coverage decreased (-0.05%) to 69.659%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
openstack/blockstorage/v2/volumes/requests.go	11	14	78.57%

Files with Coverage Reduction	New Missed Lines	%
openstack/csbs/v1/backup/results.go	3	88.89%
service_client.go	8	8.57%
openstack/compute/v2/extensions/schedulerhints/requests.go	36	56.63%
provider_client.go	50	51.12%
openstack/client.go	106	25.81%

Totals
Change from base Build 213:	-0.05%
Covered Lines:	11266
Relevant Lines:	16173

---

💛 - Coveralls

[Savasw]

@niuzhenguo This PR adds capability to delete EVS disk with cascading option so that snapshots are deleted first in order to facilitate successful volume deletion. This is useful in VBS acc tests.

[zengchen1024]

It is better to add a check before line 84, which is 'if opts != nil'. If pass a nil value for 'opts', it will raise a panic to invoke 'ToVolumeDeleteQuery' directly.

[zengchen1024]

root@chenzeng-server:/go_workspace/workspace/src/test# cat t.go
package main

type Builder interface {
To() (string, error)
}

func f1(b Builder) {
b.To()
}

func main() {
f1(nil)
}

root@chenzeng-server:/go_workspace/workspace/src/test# go run t.go

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x18 pc=0x44c172]

goroutine 1 [running]:
main.f1(0x0, 0x0)
/go_workspace/workspace/src/test/t.go:8 +0x22
main.main()
/go_workspace/workspace/src/test/t.go:12 +0x29
exit status 2

[zengchen1024]

It must check b in func f1