Add systemd service file for RHEL7

[mkrakowitzer]

Currently it works us the init script. Would be nice to use systemd service file instead.

[wouterhummelink]

This causes puppet 4.2 (PE 2015.02) to enable the service every run due to systemd responding differently for fake units

Notice: /Stage[main]/Confluence::Service/Service[confluence]/enable: enable changed 'false' to 'true'

[igalic]

fixed in #46