support security_token to authenticate with a temporary security credential

[ShiChangkuo]

What this PR does / why we need it:
fixes #1051

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

Special notes for your reviewer:

Release note:

PR Checklist

• Tests added/passed.
• Documentation updated.
• Schema updated.

Acceptance Steps Performed

$ export HW_ACCESS_KEY="0BQDG60W2070MQX8UHQ8"
$ export HW_SECRET_KEY="b15kYsk2D5xyOSJTx0UbaEdnVIk9y..."
$ export HW_SECURITY_TOKEN="gQpjbi1ub3J0aC00jB77FN3WYc3d..."
$
$ make testacc TEST='./huaweicloud' TESTARGS='-run TestAccComputeV2Instance_basic'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./huaweicloud -v -run TestAccComputeV2Instance_basic -timeout 360m -parallel 4
=== RUN   TestAccComputeV2Instance_basic
=== PAUSE TestAccComputeV2Instance_basic
=== CONT  TestAccComputeV2Instance_basic
--- PASS: TestAccComputeV2Instance_basic (162.12s)
PASS
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud       162.166s
$
$ make testacc TEST='./huaweicloud' TESTARGS='-run TestAccObsBucket_basic'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./huaweicloud -v -run TestAccObsBucket_basic -timeout 360m -parallel 4
=== RUN   TestAccObsBucket_basic
=== PAUSE TestAccObsBucket_basic
=== CONT  TestAccObsBucket_basic
--- PASS: TestAccObsBucket_basic (33.40s)
PASS
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud       33.448s