switch to tempest instead of deprecated tempest-lib
switch to tempest instead of deprecated tempest-lib

tempest-lib is deprecated and recommends us to use tempest instead.[1]

tempest actually is not required from our repository, because when
we run tempest, it must be installed and setup firstly (at least for
now). But when we generate document for our tempest test code, it is
required, so add it to test-requirements.txt.

[1]: https://github.com/openstack/tempest-lib

Change-Id: Id04f5340c27cfc84f4aba4732684a73a977ccec8

mahak/keystone,ilay09/keystone,openstack/keystone,mahak/keystone,rajalokan/keystone,openstack/keystone,cernops/keystone,openstack/keystone,mahak/keystone,rajalokan/keystone,ilay09/keystone,cernops/keystone,rajalokan/keystone,ilay09/keystone