add vpc bandwidth data source and docs

[ShiChangkuo]

Fixes #575

• add vpc bandwidth data source and docs
• Add some attributes for bandwidth resource

The acceptance testing result as follows:

$ make testacc TEST='./huaweicloud' TESTARGS='-run TestAccVpcBandWidthV2_basic'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./huaweicloud -v -run TestAccVpcBandWidthV2_basic -timeout 360m
=== RUN   TestAccVpcBandWidthV2_basic
--- PASS: TestAccVpcBandWidthV2_basic (34.21s)
PASS
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud       34.247s

$ make testacc TEST='./huaweicloud' TESTARGS='-run TestAccBandWidthDataSource_basic'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./huaweicloud -v -run TestAccBandWidthDataSource_basic -timeout 360m
=== RUN   TestAccBandWidthDataSource_basic
--- PASS: TestAccBandWidthDataSource_basic (23.58s)
PASS
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud       23.628s