IMPALA-1235: Get a new Impala Client instance every time a user reconnects.

Earlier, if the user connected via the command line, the shell would only attempt to
create a new Impala Client instance if a connection did not exist. This resulted in the
shell not connecting to the user specified Impalad.

Change-Id: I74c291256d0c063f6324b01aa7336282e6969a4e
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4392
Reviewed-by: Ishaan Joshi <ishaan@cloudera.com>
Tested-by: jenkins