Support choosing distribution/codename via $RELEASE_DISTRIBUTION
Support choosing distribution/codename via $RELEASE_DISTRIBUTION

So far we set the distribution/codename to $release.  Now it's
possible to point distribution/codename to a different value by
using $RELEASE_DISTRIBUTION.

Usage example:

 export release=0.42-update
 export RELEASE_DISTRIBUTION=jessie

will result (with default settings) in a repository under
/srv/repository/release/0.42-update/ with a setting of
"Codename: jessie" inside
/srv/repository/release/0.42-update/conf/distributions

Development sponsored by Sipwise GmbH, recorded as
MT#17701 in customers' ticket system.

tonin/jenkins-debian-glue,wikimedia/operations-debs-jenkins-debian-glue,hashar/jenkins-debian-glue,wikimedia/operations-debs-jenkins-debian-glue,webtrekk-kane/jenkins-debian-glue,mika/jenkins-debian-glue,hashar/jenkins-debian-glue,jbfavre/jenkins-debian-glue,linuxmaniac/jenkins-debian-glue,kartikm/jenkins-debian-glue,kartikm/jenkins-debian-glue,webtrekk-kane/jenkins-debian-glue,mika/jenkins-debian-glue,tonin/jenkins-debian-glue,linuxmaniac/jenkins-debian-glue,jbfavre/jenkins-debian-glue