Set run-custom-cluster-tests.sh to execute with bash
Set run-custom-cluster-tests.sh to execute with bash

Fix tests/run-custom-cluster-tests.sh to execute with bash rather than
sh so that the bash builtin commands, pushd and popd, will work.

Use #!/usr/bin/env bash so that it will run the bash in the user’s
environment rather than a hardcoded path. This is the standard used
by most of the scripts, though not all follow it.  Some do not depend
on bash only impala-shell.sh requires bash and uses a fixed path.

Change-Id: Iadb2554775fe0099f594b56df3e933ea35ed8cde
Reviewed-on: http://gerrit.cloudera.org:8080/544
Reviewed-by: Casey Ching <77bc9a9984e6786c5799d7a71768cf023120ef76@cloudera.com>
Tested-by: Internal Jenkins

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