tasks/exec: add all-roles, all-hosts keys
tasks/exec: add all-roles, all-hosts keys

'all' is ambiguous!

Signed-off-by: Sage Weil <6dd34506bbd5e58221bbb3e4732d97c91f02277b@newdream.net>

ceph/teuthology,ceph/teuthology,ktdreyer/teuthology,ktdreyer/teuthology