Update CentOS and Fedora templates to support archtectures option.

Added code to the CentOS and Fedora templates so that x86 32 bit containers
may be built on x86_64 platforms.  Like archectectures may also be trivially
used as well.

Option added is "-a {arch}".

Additionally cleaned up some bash specific logic.

Signed-off-by: Michael H. Warfield <mhw@WittsEnd.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>