Having 2 entrypoint seems redudant and complicated for end users,
so instead of rally-manage db <commands> let's use rally db <commands>
This reduces amount of code and commands needed to know to work with
Rally which is actually good.
This patch doesn't remove rally-manage just deprecates it
Change-Id: Ib35db2d22174cd3d3eee0d87358ad07a91dce0d9