Increase aws-ares cluster to 4 nodes
Allow showfast settings to be overridden

To allow the use of an alternative showfast host (for testing,
development or alternative deployment) add support for specifying
showfast host and password using:

    /tmp/env/bin/python -m perfrunner ... stats.showfast_host.\"my.showfast.couchbase.com\"

(Note the escaped quotes needed around the parameter value if it
contains dots).