Cleaned up documentation.
Cleaned up documentation.

Removed some Inktank references, mentioned the split of tasks (between
teuthology/task and chef-qa-suite/tasks), fixed some wording, removed
chef reference that no longer applies.

Did not update the teuthology-suite section at the end of this file
because further documentation changes are coming.

Fixes: #9292
Signed-off-by: Warren Usui <021fe6d8989d7bdb08c8700d8c426ea80a140097@inktank.com>

ktdreyer/teuthology,robbat2/teuthology,caibo2014/teuthology,michaelsevilla/teuthology,SUSE/teuthology,t-miyamae/teuthology,ivotron/teuthology,dmick/teuthology,ivotron/teuthology,tchaikov/teuthology,dmick/teuthology,zhouyuan/teuthology,ktdreyer/teuthology,t-miyamae/teuthology,ceph/teuthology,zhouyuan/teuthology,robbat2/teuthology,caibo2014/teuthology,ceph/teuthology,dreamhost/teuthology,tchaikov/teuthology,yghannam/teuthology,SUSE/teuthology,SUSE/teuthology,dmick/teuthology,yghannam/teuthology,dreamhost/teuthology,michaelsevilla/teuthology