Store DIB_* only env variables

Or environment variables like OS_USERNAME OS_PASSWORD will be stored
in /etc/dib_environment also. It's potentially unsafe.

Change-Id: I3a65d5bc0e4469a071db9ac23ebc82196a3f3feb

bin/disk-image-create

@@ -23,7 +23,7 @@ export LANG=C
 
 # Store our initial environment and command line args for later
 export DIB_ARGS="$@"
-export DIB_ENV=$(export)
+export DIB_ENV=$(export | grep ' DIB_.*=')
 
 SCRIPTNAME=$(basename $0)
 SCRIPT_HOME=$(dirname $0)