fixes to installer to allow it to work when running on RHEL minimal install

two changes -
1) perl isn't available on the RHEL minimal install.  It's installed by openshift in due course, but as the installer runs before that happens, use sed -i instead of perl -p -i.
2) ntp and ntpdate aren't available on the RHEL minimal install, so yum -y install them.  Do this after the initial yum -y install || exit 1 so we have confidence the yum repos are working.