suppress error unbound variable

$ grep OO_PROVIDER cluster.sh ; ./cluster.sh 
PROVIDER=$OO_PROVIDER
./cluster.sh: line 7: OO_PROVIDER: unbound variable

$ vim ./cluster.sh

$ grep OO_PROVIDER cluster.sh ; ./cluster.sh 
PROVIDER=${OO_PROVIDER:-''}
        ./cluster.sh : [create|terminate|update|list] { GCE environment tag}
        Supported environment tags:
              SUPPORTED_ENVS = %w(prod stg int tint kint test jint amint tdint lint)