acr module should use their mgmt SDK instead of including code in module

Whilst https://github.com/Azure/azure-cli/pull/859 has been merged, it includes `mgmt_acr` in the module code.

Instead, it should depend on an mgmt SDK.
When that mgmt SDK has been packaged and published, the acr module should be updated accordingly.

This requires the swagger spec to be merged first https://github.com/Azure/azure-rest-api-specs/pull/549.

FYI: @djyou, @SajayAntony, @mayurid
