CBQE-2430: Don't require a default value for test input parameters

Sometime it is useful if a test parameter is mandatory and no default
value is set. It will raise an exception is the parameter is not
specified in the test configuration.

Change-Id: I09e9a509270748b04fb7de18c5ca1f9520fb5afd
Reviewed-on: http://review.couchbase.org/42125
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Ketaki Gangal <ketakigangal@gmail.com>