Allow default tags to be set using a variable

[gps035]

In our organisation, we require a set of tags to be set on all resources. It currently isn't possible to do that on AFT created resources, and this PR allows that.

#466 (which superseeds #23) suggests merging customer defined tags with the default managed_by tag currently set on all providers. This is less than ideal for our use case, as we don't want to have that tag. We actually require a MANAGED_BY tag (all-caps).

Ideally we would be able to define the providers ourselves, but that would be a breaking change. If you are interested, I would be happy to implement that. This would allow us to have different sets of tags on each provider, which we would like, in order to have a different ACCOUNT_ID tag

[gps035]

I have performed a plan of this change against our deployed infrastructure, and the changes are as I expect.