Support air gapped installations of terraform

[lbergnehr]

When using the mixin, terraform is installed from the official https://releases.hashicorp... URL. In an air gapped environment, a user would want to install it from another source. Some possible suggestions to a solution:

1. Make it configurable from which root to install terraform, assuming the path structure is the same as the on at hashicorp's release store.
2. Make it configurable from which exact place to get the binary. The version of terraform provided as the mixin setting would be void at this point.
3. Simply make it possible to disable the installation and let the user do that on their own in the dockerfile. The mixin could provide an ENV var in the dockerfile to make it possible to still use the version information provided in the mixin settings.

[lbergnehr]

Note that it's not possible to use a .terraformrc file setting a network mirror and have that being used by the terraform provider mirrirs command.