Support setting all reprepro directories + $RELEASE_REPOSITORY

If you don't want to use the default reprepro sub directories
you can easily adjust the configuration by the newly introduced
variables:

* $REPOSITORY_OUT_DIR
* $REPOSITORY_CONF_DIR
* $REPOSITORY_DIST_DIR
* $REPOSITORY_LOG_DIR
* $REPOSITORY_DB_DIR
* $REPOSITORY_LIST_DIR
* $REPOSITORY_MORGUE_DIR

See 'man reprepro' for further details regarding the '+b' and
'+o' shortcuts.

If you also don't want to use ${REPOSITORY}/release/${release} as
the release repository for whatever reason you can change it
easily using $RELEASE_REPOSITORY now.

Thanks: Marcel Beck <nexeck@outlook.com> for the initial patches