write passwords only to serial console, lock down cloud-init-output.log (#847)

Prior to this commit, when a user specified configuration which would
generate random passwords for users, cloud-init would cause those
passwords to be written to the serial console by emitting them on
stderr.  In the default configuration, any stdout or stderr emitted by
cloud-init is also written to `/var/log/cloud-init-output.log`.  This
file is world-readable, meaning that those randomly-generated passwords
were available to be read by any user with access to the system.  This
presents an obvious security issue.

This commit responds to this issue in two ways:

* We address the direct issue by moving from writing the passwords to
  sys.stderr to writing them directly to /dev/console (via
  util.multi_log); this means that the passwords will never end up in
  cloud-init-output.log
* To avoid future issues like this, we also modify the logging code so
  that any files created in a log sink subprocess will only be
  owner/group readable and, if it exists, will be owned by the adm
  group.  This results in `/var/log/cloud-init-output.log` no longer
  being world-readable, meaning that if there are other parts of the
  codebase that are emitting sensitive data intended for the serial
  console, that data is no longer available to all users of the system.

LP: #1918303

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