Stop copying ssh system keys and check folder permissions (#956)

In /etc/ssh/sshd_config, it is possible to define a custom
authorized_keys file that will contain the keys allowed to access the
machine via the AuthorizedKeysFile option. Cloudinit is able to add
user-specific keys to the existing ones, but we need to be careful on
which of the authorized_keys files listed to pick.
Chosing a file that is shared by all user will cause security
issues, because the owner of that key can then access also other users.

We therefore pick an authorized_keys file only if it satisfies the
following conditions:
1. it is not a "global" file, ie it must be defined in
   AuthorizedKeysFile with %u, %h or be in  /home/<user>. This avoids
   security issues.
2. it must comply with ssh permission requirements, otherwise the ssh
   agent won't use that file.

If it doesn't meet either of those conditions, write to
~/.ssh/authorized_keys

We also need to consider the case when the chosen authorized_keys file
does not exist. In this case, the existing behavior of cloud-init is
to create the new file. We therefore need to be sure that the file
complies with ssh permissions too, by setting:
- the actual file to permission 600, and owned by the user
- the directories in the path that do not exist must be root owned and
  with permission 755.

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