IMPALA-3788: Fix Kudu ReadMode flag checking
IMPALA-3788: Fix Kudu ReadMode flag checking

Commit 0d4bdc1 to add flag --kudu_read_mode wasn't checking
the value properly; so the behavior was reversed. This
became apparent immediately in stress testing, though there
isn't a good way to validate this in EE or unit tests. I
added some logging which I used to verify the correct
behavior manually in the minicluster.

Change-Id: I843842560844732505af53ea360a3a9ccbab04a9
Reviewed-on: http://gerrit.cloudera.org:8080/5402
Reviewed-by: Matthew Jacobs <df2fa7c39d141dc5dc5589fed74180e757ed6c9a@cloudera.com>
Tested-by: Internal Jenkins

cloudera/Impala,cloudera/Impala,michaelhkw/incubator-impala,michaelhkw/incubator-impala,cloudera/Impala,cloudera/Impala,cloudera/Impala,michaelhkw/incubator-impala,michaelhkw/incubator-impala,cloudera/Impala,michaelhkw/incubator-impala,michaelhkw/incubator-impala,michaelhkw/incubator-impala,cloudera/Impala