Use set-os-type to set default NODE_DIST

Adds a new script called set-os-type which can be used
to export TRIPLEO_OS_FAMILY and TRIPLEO_OS_DISTRO in
your current environment.

Updates the devtest.sh script to call set-os-type and
then we can use the TRIPLEO_OS_DISTRO as the default
value for NODE_DIST.

Change-Id: I504b4c41a2fec6958fe16c8b2a2dcff6b719f70a