serf package has invalid systemd service and configuration

[Rowan James]

Description: Ubuntu 16.04.3 LTS
Release: 16.04

serf:
  Installed: 0.6.4~ds1-1
  Candidate: 0.6.4~ds1-1
  Version table:
 *** 0.6.4~ds1-1 500
        500 http://au.archive.ubuntu.com/ubuntu xenial/universe amd64 Packages
        100 /var/lib/dpkg/status

Expected:
installing serf (apt install serf) results in a working serf installation

Actual:
installing serf results in systemd and serf errors

--
The install service definition (/lib/systemd/system/serf.service) has invalid lines, causing the service to fail to start.

1.
ExecPreStart= should read ExecStartPre=
(this is fixed in hashicorp/serf GitHub repository as a typo)

2.
ExecStart= gives the wrong path, the executable is not installed under /usr/local/bin/serf, but under /usr/bin/serf

--
3.
The specified configuration directory /etc/serf contains an example configuration (serf.json.example) but no default configuration (serf.json) - causing the service to fail on launch. The example configuration fails on my system due to hard-coded interface name "eth0" not being present, and specifying the unsuitable "wan" profile.

A suitable "defaults" configuration for my is an empty JSON object "{}" in /etc/serf/serf.json