Added workspace support

[nmiculinic]

This is another way to solve #247, to have per tf instance workspace support. This is especially important for our usecase where we concurrently run terraform apply over the same directory, but different workspaces

[hashicorp-cla]

All committers have signed the CLA.

[sergiorivera50]

Is this dead?

[fsaintjacques]

@radeksimko this is a blocker for us. We would like to run tfexec concurrently for different workspaces. Depending on the state of .terraform/environment makes this impossible without a lock. This PR would allow us to do it concurrently for independent workspaces.