Fix create-user.sh to work with bash 4
Fix create-user.sh to work with bash 4

This script has `set -u` at the top, and was therefore erroring with:

./scripts/create-user.sh: line 181: uaa_uuid: unbound variable

For some reason this wasn't causing an issue with bash 3 (as included in
OS X). I presume bash 3 is not complaining because this variable is
declared as 'local' at the top of this function.

alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf