In previous GPDB version, the distribution keys may be changed implicitly
when creating a unique index on a hash-distributed empty table.
```SQL
create table foo(a int, b int) distributed by(a);
create unique index on foo(b);
-- now, foo is hash distributed by b, not by a
```
It might be useful(maybe) to avoid changing the distribution keys. However,
on the other side, it's crazy if the user doesn't notice the NOTICE message
like, "NOTICE:  updating distribution policy to match new UNIQUE index".

What's worse, this behavior could bring data inconsistency. See,
```SQL
create table foo(a int, b int) distributed by(a);
insert into foo select i,i from generate_series(1,5)i;

create table foopart (i int4, j int4) distributed by (i) partition by
        range (i) (start (1) end (3) every (1));
create unique index on foopart_1_prt_1 (j);
insert into foopart values(1,2),(2,1);
```
The data inconsistency is
```
gpadmin=# select gp_segment_id, * from foopart_1_prt_1;
 gp_segment_id | i | j
 ---------------+---+---
             1 | 1 | 2
 (1 row)

gpadmin=# select * from foo f, foopart_1_prt_1 p where f.a = p.j;
 a | b | i | j
 ---+---+---+---
 (0 rows)
```

Implicitly changing the distribution keys is not very useful, but harmful.
This PR disables changing the distribution keys when creating a unique index.
Reviewed-by: NHubert Zhang <hzhang@pivotal.io>