bin/createdb-if-not-exists: use instead of dropdb

Developing locally against Dashboard uses the `ledger` database.
Running `$CHAIN/sdk/{java,node,ruby}/bin/tests` drops that database.

Instead of dropping the database and then creating it,
check to see if it doesn't exist, and then create it.

Extract to a script for clarity.

Closes #3559

Author: Dan Croak <dan@statusok.com>
Date: Wed May 2 12:08:22 2018 -0700
upstream:a1cfec659f968c7984f7b546f37a590731bae64c

Author: croaky

bin/tests

@@ -1,6 +1,6 @@
 #!/bin/bash
 set -xeo pipefail
-dropdb --if-exists ledger; createdb ledger
+createdb-if-not-exists ledger
 cd $CHAIN/sdk/node
 npm install --scripts-prepend-node-path
 with-ledgerd npm test --scripts-prepend-node-path