Which terraform

[jcollado]

Description

Check that terraform binary is available before trying to use it.

Motivation and Context

Remove error messages when a .terraform directory is present, but the terraform binary is not available.

Closes #534

Screenshots (if appropriate)

How Has This Been Tested

I have used the code in this with/without terraform being present, and it worked as expected.

• [x] I have tested using Linux.
• [ ] I have tested using MacOS.

Checklist

• [x] I am ready to update the wiki accordingly.
• [x] I have updated the tests accordingly.

[jcollado]

A couple of comments regarding the checklist:

• I believe that the wiki doesn't need to be updated.
• I've updated the test cases so that they pass. However, I don't know what would be the right way to add a test to verify the situation when the terraform command is not found.