Add epel and rdo-release to DIB_COMMON_ELEMENTS

Since we include tripleo-image-elements as a common element, we need
epel there too so the dependencies can be satisfied.
Be smarter about default NODE_DIST

Instead of always defaulting to Fedora, default to whatever the host
distro is.
Heat version to latest stable-juno

We need latest stable-juno version of heat, which has for example
fixed stack-update.