OTP releases: only set name and distribution type if not set already

Fixes not being able to run two OTP releases on one machine because of
the name conflict.

rel/env.sh.eex

@@ -8,5 +8,5 @@
 # fi
 
 # Set the release to work across nodes
-export RELEASE_DISTRIBUTION=name
-export RELEASE_NODE=<%= @release.name %>@127.0.0.1
+export RELEASE_DISTRIBUTION="${RELEASE_DISTRIBUTION:-name}"
+export RELEASE_NODE="${RELEASE_NODE:-<%= @release.name %>@127.0.0.1}"