Fix the systemd unit file for clamd@.service if needed

Summary: Resolves T997

Test Plan:

1. Install a Kolab server on CentOS 7
2. Set up Kolab
3. Expect a running instance of the clamd@amavisd service

Reviewers: PyKolab Developers, vanmeeuwen

Reviewed By: PyKolab Developers, vanmeeuwen

Maniphest Tasks: T997

Differential Revision: https://git.kolab.org/D86