[lbaas_v2/monitors] make expected_codes be optional for HTTP(S) types

[ShiChangkuo]

What this PR does / why we need it:

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:

$ go test -v
=== RUN   TestListHealthmonitors
--- PASS: TestListHealthmonitors (0.00s)
=== RUN   TestListAllHealthmonitors
--- PASS: TestListAllHealthmonitors (0.00s)
=== RUN   TestCreateHealthmonitor
--- PASS: TestCreateHealthmonitor (0.00s)
=== RUN   TestRequiredCreateOpts
--- PASS: TestRequiredCreateOpts (0.00s)
=== RUN   TestGetHealthmonitor
--- PASS: TestGetHealthmonitor (0.00s)
=== RUN   TestDeleteHealthmonitor
--- PASS: TestDeleteHealthmonitor (0.00s)
=== RUN   TestUpdateHealthmonitor
--- PASS: TestUpdateHealthmonitor (0.00s)
=== RUN   TestDelayMustBeGreaterOrEqualThanTimeout
--- PASS: TestDelayMustBeGreaterOrEqualThanTimeout (0.00s)
PASS
ok      github.com/huaweicloud/golangsdk/openstack/networking/v2/extensions/lbaas_v2/monitors/testing   0.018s

[coveralls]

Pull Request Test Coverage Report for Build 1234

• 4 of 4 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 68.336%

---

Totals
Change from base Build 1231:	0.02%
Covered Lines:	11654
Relevant Lines:	17054

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 1234

• 4 of 4 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 68.336%

---

Totals
Change from base Build 1231:	0.02%
Covered Lines:	11654
Relevant Lines:	17054

---

💛 - Coveralls