Add rhel-7-undercloud-packages.json
Choose the correct json file depending on your OS
Merge pull request #79 from rwsu/pick-rhel-json

Choose the correct json file depending on your OS
Heat version to latest stable-juno

We need latest stable-juno version of heat, which has for example
fixed stack-update.
Also adding a note about variable needed to be set, so this
reporefs are actually used.