Use BaremetalArch parameter instead of sed.

Updates heat stack-create command for the undercloud so that
it makes use of the BaremetalArch parameter. This allows
us to remove the broken sed statement which was trying to
do the same thing (but used the wrong substitution string anyway).

Change-Id: I2dfa238352919cae2a6989112fccf065c0f6aa8c
Merge "Use BaremetalArch parameter instead of sed."