[terraform]: allow to override service type

[kayrus]

Resolves #228

[kayrus]

cc @nikParasyr

[kayrus]

@pierreprinetti @mandre @EmilienM kindly ping

[pierreprinetti]

Why do we even have a Terraform directory in utils? What is its purpose?

I have nothing against merging this, but I think we should have a migration plan for moving this code to the Terraform provider, if it's needed there

[kayrus]

Why do we even have a Terraform directory in utils? What is its purpose?

initially this was in the openstack terraform project, but this code is used in other terraform related projects that use non-vanilla openstack services. at some point we decided to move it into the gophercloud/utils to reduce the dependency tree.

[pierreprinetti]

Do you think that having all Gophercloud functions accept ServiceClient as an interface (as proposed here) would solve this use-case?

[kayrus]

In theory yes, but the code base should contain some extra logic. Anyway, I don't think that terraform helper code will be rewritten after introducing the client interface.

[pierreprinetti]

I don't think that terraform helper code will be rewritten after introducing the client interface

Indeed, that's fair. I'll keep my spring-cleaning instincts under control for the time being