Fix the way to get tempest.conf in README.rst

We're using oslo-config-generator to generate a config file other than
copy the tempest.config.sample. We should update the description in
qucikstart of README.rst.

Change-Id: I5c7c3f9041da3e5c3840b440bba6e614c23fdf8a
Closes-Bug: #1481635
1 file changed