Use exec for the terraform command to avoid problem in SIGTERM

[pecigonzalo]

tfenv-exec is used by the terraform stub as exec but its not done on the tfenv-exec command, this causes that TERM and other signals land on bash instead of terraform which should not be the case and could lead to other problems as the "main" process is bash instead of terraform.
It should use exec to replace the bash process

[pecigonzalo]

@Zordrak Any comments on this?

[Zordrak]

LGTM