tlsdated: support multiple failover hosts

Swap over to using 'source' stanzas in the config file (see tlsdated.conf for an
example), and cycle through them as we make tlsdate attempts.

TODO: changelog, man page, etc updates

Signed-off-by: Elly Fong-Jones <ellyjones@chromium.org>
Merge pull request #69 from elly/multi-host

tlsdated: support multiple failover hosts; epic, you rock!