Create "technomancy.syme.in" DNS entries

[technomancy]

There's not a 1:1 mapping from users to instances of course, so we might want to further subdivide it like technomancy.4.syme.in or something.

[technomancy]

All the functionality for this exists; just need to tie it into the instance launch cycle and decide how to select subdomains.

[technomancy]

Discussion from IRC:

<technomancy> maio: heh; missed you last night. so yeah, I think there's
              really no reason you can't re-use instances; better to
              manually bootstrap an existing instance for a new project
              than wait for a new one to boot.  [08:21]
<technomancy> maio: OTOH, do we enforce this limitation? if not then we
              have to be clever and fall back to the IP if someone launches
              a second instance without halting the first.  [08:22]
<technomancy> the problem with username.project.syme.in is that project is
              multi-segmented
<technomancy> so it'd have to be technomancy.technomancy.syme.syme.in which
              is even more annoying than the IP =)  [08:23]
<technomancy> I guess we could assume each user is not going to hack on
              multiple forks of a project I guess

Multiple DNS entries per user is simpler conceptually, but I think it's not as good of an experience for the end user.

[technomancy]

So the problem with re-using DNS entries for different instances is that you have to either turn off strict hostkey checking or clear out entries from ~/.ssh/known_hosts, both of which are a drag.