Apache does not start on `systemctl start` with the rest of foreman.target

[qcjames53]

When running systemctl start foreman.target, Apache (httpd) does not start on it's own. I attempted to include this as a part of the fix in #536 but biffed the implementation.

Correct src/roles/httpd/tasks/main.yml such that Apache remains part of the foreman.target, but remove the faulty [Install] block ignored by systemd. Use systemctl add-wants foreman.target httpd.service as a replacement, as this is standard practice. Ensure that systemd actions work on the foreman.target.

[qcjames53]

PR: #561