Allow option to specific CREATE INDEX to be CONCURRENTLY or NONCONCURRENTLY for target database YugabyteDB

[ssherwood]

YugabyteDB defaults index creation to CONCURRENTLY. This is most appropriate for on-line (live) migrations. However, for offline migrations, it may be desirable to have YugabyteDB use NONCONCURRENTLY instead.

This should be an optional configuration option when generating the CREATE INDEX output for YugabyteDB.