upstart: Don't emit "started" event until docker.sock is available

Fixes #6647: Other upstart jobs that depend on docker by specifying
"start on started docker" would often start before the docker daemon was
ready, so they'd fail with "Cannot connect to the Docker daemon" or
"dial unix /var/run/docker.sock: no such file or directory".

This is because "docker -d" doesn't daemonize, it runs in the
foreground, so upstart can't know when the daemon is ready to receive
incoming connections. (Traditionally, a daemon will create all necessary
sockets and then fork to signal that it's ready; according to @tianon
this "isn't possible in Go"[1]. See also [2].)

Presumably this isn't a problem with systemd init with its socket
activation. The SysV init scripts may or may not suffer from this
problem but I have no motivation to fix them.

This commit adds a "post-start" stanza to the upstart configuration
that waits for the socket to be available. Upstart won't emit the
"started" event until the "post-start" script completes.[3]

Note that the system administrator might have specified a different path
for the socket, or a tcp socket instead, by customising
/etc/default/docker. In that case we don't try to figure out what the
new socket is, but at least we don't wait in vain for
/var/run/docker.sock to appear. If for some unforeseen reason we do end
up in an infinite loop, you'll be able to see the "Waiting for
/var/run/docker.sock" debug output in /var/log/upstart/docker.log.

I considered using inotifywait instead of sleep, but it isn't worth
the complexity & the extra dependency.

[1] https://github.com/docker/docker/issues/6647#issuecomment-47001613
[2] https://code.google.com/p/go/issues/detail?id=227
[3] http://upstart.ubuntu.com/cookbook/#post-start

Signed-off-by: David Röthlisberger <david@rothlis.net>