support per-role distribution specification
See also this thread: https://spinics.net/lists/ceph-devel/msg42762.html
Extend the "roles" array to accept a distribution specification, e.g.:
roles: - daemons: ["osd.0", "mon.a", ...] os: type: ubuntu version: 16.04 kernel: testing - daemons: ["client.0"] os: type: rhel version: 7.5 kernel: distro
Teuthology should continue to accept the old specification as well:
roles: - [osd.0, ...] - [osd.1, ...]
The driving need for this currently is that we want to continue testing older kernels like RHEL7 but this is difficult when Octopus no longer has builds for RHEL7/CentOS7.