promote the firewall disabling feature

the hdp element would disable firewalls in images. other elements are
growing similar functionality. this change makes the functionality
available as an element.

Change-Id: Ib8315d6479b88a6378b58733223e9238435dfb51
Fix platform check in diskimage-create.sh

'set -e' causes the script to exit when 'grep CentOS' fails, which
meant the subsequent test was never reached. the subsequent test was
also backwards, using -z (test for zero length) instead of -n (test
for non-zero length).

Change-Id: Ie894a0ef4d785af6b9cb4a9f690bc6752092e655
Remove all iptables rules that are set in base image

Change-Id: If92e0e7a6ae667b70eb7d0df5d6534a943032c42
Merge "promote the firewall disabling feature"
Merge "Remove all iptables rules that are set in base image"
Merge "Fix platform check in diskimage-create.sh"