Do not create GROUP_OU and USER_OU

[oehrlis]

If variables GROUP_OU and USER_OU are not set the corresponding OU's should not be created

[oehrlis]

Add simple check if certain variables are defined like

# - check prerequisites --------------------------------------------------------
# check mandatory variables
[ -z ${OUD_INSTANCE_HOME} ] && echo "- skip $(basename $0), variable OUD_INSTANCE_HOME not set" && exit
[ -z ${PWD_FILE} ]          && echo "- skip $(basename $0), variable PWD_FILE not set"          && exit
[ -f ${PWD_FILE} ]          && echo "- skip $(basename $0), missing password file ${PWD_FILE}"  && exit
[ -z ${HOST} ]              && echo "- skip $(basename $0), variable HOST not set"              && exit
[ -z ${PORT} ]              && echo "- skip $(basename $0), variable PORT not set"              && exit
[ -z ${PORT_SSL} ]          && echo "- skip $(basename $0), variable PORT_SSL not set"          && exit
[ -z ${PORT_ADMIN} ]        && echo "- skip $(basename $0), variable PORT_ADMIN not set"        && exit
[ -z ${PORT_REST_ADMIN} ]   && echo "- skip $(basename $0), variable PORT_REST_ADMIN not set"   && exit
[ -z ${PORT_REST_HTTP} ]    && echo "- skip $(basename $0), variable PORT_REST_HTTP not set"    && exit
[ -z ${PORT_REST_HTTPS} ]   && echo "- skip $(basename $0), variable PORT_REST_HTTPS not set"   && exit
[ -z ${DIRMAN} ]            && echo "- skip $(basename $0), variable DIRMAN not set"            && exit
[ -z ${BASEDN} ]            && echo "- skip $(basename $0), variable BASEDN not set"            && exit