Advise folk to sit in $TRIPLE_ROOT not incubator.
Advise folk to sit in $TRIPLE_ROOT not incubator.

The incubator directory shouldn't be assumed to be the working
directory as our tools need to be installable.

Telling folk to cd to it makes it special and increases the chance of
accidentally making code assume it is the working directory.

Change-Id: Ib09407e1ce97fa5fd12d4b7387e57d04e9f002d3

rdo-management/tripleo,rdo-management/tripleo