systemd: fix exabgp paths

README and SysV init scripts are expecting exabgp to be in
/usr/sbin. We fix the path in systemd service file. Moreover, SysV
init script looks at ExaBGP configuration in
/etc/exabgp/exabgp.conf. We also fix this for systemd.
Merge pull request #63 from vincentbernat/fix/systemd-path

systemd: fix exabgp paths