This allows users to override - at a top-level - the DOKKU_ROOT, DOKKU_LIB_ROOT, and other
configuration variables. If specified, this should begin to enable users to move their
Dokku installations to mounted drives.
Note that items installed from packages may not respect these paths, so further work may be in order.
Closes#2958
The centos rpm packages should be installable on centos once this is merged and released. However, Dokku will not maintain an Rhel-specific repository as there is no way for us to feasibly test it.
Closes#3046
