Allow jenkins to sudo as any user.

Jenkins needs to sudo as stack and tempest to properly run devstack jobs
and may need to act on behalf of other users in the future. Rather than
listing root,stack,tempest in the sudoer file just let jenkins sudo as
any user.

Fixes-bug: 1290669
Change-Id: I4a5c6b7ead717475d6334a7ef3121271aca83f5a