Make good use of TRIPLEO_OS_{FAMILY,DISTRO} where useful.

This is an attempt to remove duplicated code intended to check
for the running OS; I replaced that with simple checks against
the TRIPLEO_OS_* variables (set by devtests_variables in the
early stages). Documentation is updated a bit to avoid confusion
between TRIPLEO_OS_* and NODE_DIST.

Change-Id: If494f3d5165c89200ac5c965c601f6f67645bae2
Merge "Make good use of TRIPLEO_OS_{FAMILY,DISTRO} where useful."