Clarify the requirements for the TRIPLEO_ROOT variable

This change is meant to clarify the requirements for the
TRIPLEO_ROOT variable by describing its usage in the early
parts of the documentation.

It also introduces a check intended to support existing
workflows where people intentionally leave it unset.

Change-Id: I99b11f487da0db09eb2ee517d6a42744abc1b461
Merge "Clarify the requirements for the TRIPLEO_ROOT variable"