Added option to specify the scale factor for pytest

This allows execution of tests on a cluster with multiple scale factors.

For example:
py.test <test file> --impalad <cluster ip>:21000 --scale_factor 300gb

Change-Id: I5230a6ef354def44b984eab2ac8a01989b9a471c
Reviewed-on: http://gerrit.ent.cloudera.com:8080/3051
Reviewed-by: Taras Bobrovytsky <tbobrovytsky@cloudera.com>
Tested-by: jenkins