qa/tasks/tempest.py: always write str is value of options

qa/tasks/tempest.py: always write str is value of options

in Python2, ConfigParser is almost the same as RawConfigParser, which
allows set non-string values, but in Python3, ConfigParser.set() only
accepts strings as value of option.

since we do not use "cpar" as an internal storage for options, it does
not matter what type of options we set using ConfigParser as long as it
can be consumed by tempest. boolean settings are translated to "true" or
"false". see also
https://docs.openstack.org/tempest/latest/sampleconf.html

Signed-off-by: Kefu Chai <kchai@redhat.com>