Use lock file when add or remove keys from authorized_keys file

This prevents concurrent modification of authorized_keys file

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Mention lock file in changelog

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Merge branch 'authorized_keys_lock' into 'master'

Authorized keys lock