Do not overwrite shell if user already exists.
Make `sudo` keeping env-var SSH_CLIENT

In some environments, /etc/sodoers contains an entry like
`Defaults  env_reset` which will remove most environment
variables. But other debops plays require SSH_CLIENT to
be set. So add an entry to /etc/sudoers.d/admin to keep
this variable.
Fix patterns: avoid ambiguous results.

The regexp for adding the admins-group must be space-terminated,
otherwise it could be ambiguous. While this may be superfluous,
we'll be on the save side when doing this.
Fix comment about usage.
Fix #13: ensure /etc/sudoers includes /etc/sudoers.d
Merge pull request #49 from htgoebel/master

Some enhancements to bootstrap.yml