Add datasets and validators for security fuzz testing with tempest-lib

This patch adds a few helper classes to be used for security fuzz tests: https://review.openstack.org/#/c/216303/
There is a separate patch that includes the datasets and validators for various fuzz types: https://review.openstack.org/#/c/237263/2

The classes here are used to package "fuzz strings" into datasets that can be used in parameterized functional test cases. Currently, it can verify that HTTP responses to requests-lib style and tempest-lib style REST clients do not include indicators of vulnerability such as 500 errors from the server, or "root:" in the response to a request attempting to read /etc/passwd.