[ENHANCEMENT] Adding an argument to set the terraform.workspace

[rsareth]

** Description **
It is related to a previous demand: #305

Here, the idea is to allow to test different values when we want to name resources with the terraform workspace. By default, you set at default.

Just like you did for handling the tfvars, you could add an argument. An idea of usage:

$ regula run --terraform-workspace dev .

What do you think ?

Thank you in advance

Rasmey