Fix comments and typos in db/api, orchestrator/api and cmd/main

Fix and complete the comments in rally/db/api.py, rally/orchestrator/api.py,
and rally/cmd/main.py, and fix some typos when reading the code.

Change-Id: Ia4ba1233ec5988f973a9285b3964fbe7fbed6c35
Signed-off-by: Shane Wang <shane.wang@intel.com>