Can't restart a container

[BirgerK]

Doing:

• start a container, everything is fine
• stop the container
• start the container again

expectation:

• container starts like the first time

what happens:

• the container can't start because of useradd: user 'collectd-docker-collector' already exists

Seems like this line in the start-script isn't fine.
I guess it will be all fine if the script checks for the existing user.

[BirgerK]

Pull-Request is provided. Please take a look.

[bobrik]

Thanks!