rules:  use dh_python2

Signed-off-by: Gary Lowell  <gary.lowell@inktank.com>
build scripts:  Update for release builds.

Allow alternate signing keyid to be specified in environment
variable.  For debian add backport tags to release version
numbers if release flag is set.

Signed-off-by: Gary Lowell  <gary.lowell@inktank.com>