Expose the `Airy Core` version variable in root module

[armanjindal]

In the module for airy-core we have a variable to specify the version of airy as var.core_version. This variable, however, is not exposed in the root module. If unspecified, the module default grabs the latest version via an HTTP request. This lack of control may cause clients to upgrade their deployments without being aware and having no mechanism to fix the airy-core version.