[Feature] Server group supports members updation

[Lance52259]

What this PR does / why we need it:
According to server group API, server group supports members add and remove operation.

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

NONE

Special notes for your reviewer:

Release note:

1. sdk supports members add and remove operation.
2. add unit test function.

Unit tests

go test -v
=== RUN   TestAddMember
--- PASS: TestAddMember (0.00s)
=== RUN   TestRemoveMember
--- PASS: TestRemoveMember (0.00s)
PASS
ok      github.com/huaweicloud/golangsdk/openstack/ecs/v1/cloudservers/testing  0.011s

[coveralls]

Pull Request Test Coverage Report for Build 1138

• 13 of 16 (81.25%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.01%) to 68.047%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
openstack/compute/v2/extensions/servergroups/requests.go	10	13	76.92%

Totals
Change from base Build 1136:	0.01%
Covered Lines:	11523
Relevant Lines:	16934

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💛 - Coveralls