add elb_v2 loadbalancers

[Jason-Zhang9309]

What this PR does / why we need it:

add elb_v2 loadbalancers

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:

Test results:

go test -run Test -v
=== RUN   TestListLoadbalancers
--- PASS: TestListLoadbalancers (0.00s)
=== RUN   TestListAllLoadbalancers
--- PASS: TestListAllLoadbalancers (0.00s)
=== RUN   TestCreateLoadbalancer
--- PASS: TestCreateLoadbalancer (0.00s)
=== RUN   TestRequiredCreateOpts
--- PASS: TestRequiredCreateOpts (0.00s)
=== RUN   TestGetLoadbalancer
--- PASS: TestGetLoadbalancer (0.00s)
=== RUN   TestGetLoadbalancerStatusesTree
--- PASS: TestGetLoadbalancerStatusesTree (0.00s)
=== RUN   TestDeleteLoadbalancer
--- PASS: TestDeleteLoadbalancer (0.00s)
=== RUN   TestUpdateLoadbalancer
--- PASS: TestUpdateLoadbalancer (0.00s)
=== RUN   TestCascadingDeleteLoadbalancer
--- PASS: TestCascadingDeleteLoadbalancer (0.00s)
PASS
ok      github.com/huaweicloud/golangsdk/openstack/networking/v2/extensions/elb_v2/loadbalancers/testing        0.019s

[coveralls]

Pull Request Test Coverage Report for Build 1224

• 95 of 102 (93.14%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.1%) to 68.32%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
openstack/elb/v2/loadbalancers/results.go	32	34	94.12%
openstack/elb/v2/loadbalancers/requests.go	54	59	91.53%

Totals
Change from base Build 1220:	0.1%
Covered Lines:	11654
Relevant Lines:	17058

---

💛 - Coveralls