add compute instance data source and docs

[ShiChangkuo]

The testing result as follows:

$ make testacc TEST='./huaweicloud' TESTARGS='-run TestAccComputeInstanceDataSource_basic'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./huaweicloud -v -run TestAccComputeInstanceDataSource_basic -timeout 360m -parallel 4
=== RUN   TestAccComputeInstanceDataSource_basic
=== PAUSE TestAccComputeInstanceDataSource_basic
=== CONT  TestAccComputeInstanceDataSource_basic
--- PASS: TestAccComputeInstanceDataSource_basic (171.72s)
PASS
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud       171.756s