[terraform] Support arbitrary arguments

[steinybot]

So that the orb doesn't have to be updated every time the terraform parameters change, it would be good if it were possible to provide arbitrary arguments to terraform.

This could be done instead of #113.

[bamarch]

If we looked at doing this, and did not implement "lock_timeout" as an explicit parameter, think of the following problem:

The user specifies say lock-timeout=30s for plan and apply, passed in as a arbitrary argument to each.

How would the orb know to use this parameter for init (called implicitly) as well?

The commands all have a different set of supported parameters (init/plan/apply etc...) so the arbitrary args couldn't be applied everywhere TF is called by the orb.

#113 looks good to me - I think the parameters should be evaluated and added as the need arises - the arbitrary way makes something complex appear simple and might lead to issues imo

[steinybot]

That is a good point. I'm not too bothered by not having this if the forwards compatibility isn't worth the hassle.

One way that I can think to workaround the problem would be to have separate parameters for each command such as init_parameters and plan_parameters.