IMPALA-1033: Remove SOURCE keyword; very common identifier

The SOURCE keyword was introduced for DATA SOURCE ddl commands, but
it is also a very common identifier. This removes the SOURCE and
SOURCES keywords and instead uses DATASOURCE and DATASOURCES.

Change-Id: Ic6c2897d1e23efa169aa8787752fe4aa2bb125d5
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2895
Reviewed-by: Matthew Jacobs <mj@cloudera.com>
Tested-by: jenkins