Add support for custom working directory

[ThorstenHans]

With this PR, users can specify a custom working directory for theit Terraform project.

• Tests
• Docs (Readme)

[carolynvs]

I have removed the comment that this fixes #16. For #16 we should first update the builder library so that it supports changing the workding directory, and then use that in the terraform mixin.

getporter/porter#1403

[vdice]

I'm unclear whether we want this change even if/when we add changes to fix #16 . The changeset here looks good to me, if we wish to proceed with both.

[carolynvs]

Now that we have built-in support for setting the working directory on a mixin, here's how a mixin can support this too:

1. Update the dependency on porter in their go.mod file to v0.38.0+
2. Add a field "dir" to the mixin step instruction, just like the exec mixin
   https://github.com/getporter/porter/blob/aa542e4cefe02605ffc9667b8af65cbdcccf022d/pkg/exec/action.go#L102
3. Implement the updated executable step interface
   https://github.com/getporter/porter/blob/aa542e4cefe02605ffc9667b8af65cbdcccf022d/pkg/exec/action.go#L138-L140

[vdice]

@ThorstenHans now that it is possible to add support in this mixin for the standardized method of setting the working dir per step, do we still wish to add this functionality at the mixin config level?

[ThorstenHans]

Obviously, the mixin should adopt the feature from porter instead of adding support for working directory by itself.

[ThorstenHans]

Should I create a dedicated PR (for updating the porter dep) and add an example there or could I reuse this PR instead

[vdice]

@ThorstenHans Although I might have a slight preference towards the creation of a new PR, I'll leave it up to you - whichever is most convenient.

[ThorstenHans]

@vdice that's fine for me I'll close this one 👍