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).
Allow cbmonitor settings to be overridden
Allow seriesly settings to be overridden