support iec vpc resource and docs

[ShiChangkuo]

add new resource huaweicloud_iec_vpc to manage an IEC VPC resource within HuaweiCloud.
the testing result as follows:

$ make testacc TEST='./huaweicloud' TESTARGS='-run TestAccIecVpcV1_basic'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./huaweicloud -v -run TestAccIecVpcV1_basic -timeout 360m -parallel 4
=== RUN   TestAccIecVpcV1_basic
=== PAUSE TestAccIecVpcV1_basic
=== CONT  TestAccIecVpcV1_basic
--- PASS: TestAccIecVpcV1_basic (26.29s)
PASS
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud       26.325s
$
$ make testacc TEST='./huaweicloud' TESTARGS='-run TestAccIecVpcV1_customer'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./huaweicloud -v -run TestAccIecVpcV1_customer -timeout 360m -parallel 4
=== RUN   TestAccIecVpcV1_customer
=== PAUSE TestAccIecVpcV1_customer
=== CONT  TestAccIecVpcV1_customer
--- PASS: TestAccIecVpcV1_customer (26.56s)
PASS
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud       26.595s