Use authorized_keys lock when add new key to file

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Create keys fixtures in test

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

Use lock file when add new entries to authorized_keys

Fixes #21