Make improvements to sync-gateway perf test
1. Add parameter to turn on verbose logging from cluster_spec.
2. Add warn_only=true to run command.
3. getting P99 every 5 minutes during the test run so we can see P99 displayed throughout test run.
4. Add num_gateways to allow change of 2 and 3 gateways without using a different cluser spec
5. Exit the program if fail to start sync-gateway before running sgw_test_info.sh

Change-Id: I7e447df02141baea2a345dccf2a65c08333836f5
Reviewed-on: http://review.couchbase.org/36977
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Li Yang <lichenyang2@gmail.com>