Work item to make GSI index as the DEFAULT INDEXING method for Couchbase/Sherlock.

Currently the two following statements are equivalent.
> create index i1 on customer(c1,c2);
> create index i2 on customer(c1,c2) using view;

To create GSI index, you've to explicitly name it.
> create index i2 on customer(c1,c2) using gsi;

For Sherlock----->
Make gsi index as the DEFAULT indexing method from N1QL.
> create index i1 on cust-omer(c1,c2);
> create index i2 on customer(c1,c2) using gsi;

This also affects the default method for
CREATE PRIMARY INDEX...
DROP PRIMARY INDEX
DROP INDEX