Debian 9.7 - missing sysvinit support

Found pretty much same issue about this https://github.com/digitalocean/do-agent/issues/35, but opening new one as it's closed and can't be re-opened easily.

## Describe the problem

Can't install do-agent on Debian 9.7 with sysvinit ini system.

## Steps to reproduce

Have Debian 9.7 with sysvinit and try to install do-agent. Throws this error:

```
boa:~# sudo ./install.sh
Cleaning up old sources...OK
Verifying machine compatability...OK
Verifying compatability with script...OK
Reading package lists... Done
Building dependency tree
Reading state information... Done
Package 'do-agent' is not installed, so not removed
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
Installing apt repository...
Installing gpg key...OK
Selecting previously unselected package do-agent.
(Reading database ... 70509 files and directories currently installed.)
Preparing to unpack .../do-agent_3.5.5_amd64.deb ...
Unpacking do-agent (3.5.5) ...
Setting up do-agent (3.5.5) ...
Detecting SELinux
SELinux not enforced
Unknown init system. Exiting...
dpkg: error processing package do-agent (--configure):
subprocess installed post-installation script returned error exit status 1
Errors were encountered while processing:
do-agent
E: Sub-process /usr/bin/dpkg returned an error code (1)
```

## Expected behavior

do-agent should support all init systems - systemd, sysvinit, upstart. Seems that it's only supporting systemd and upstart. It is seen when viewing this code part - https://github.com/digitalocean/do-agent/blob/7977cee907ef63de9584969eed3f0fab8edd9363/packaging/scripts/after_install.sh#L24

## System Information
Distribution and version: Debian 9.7 x64

### do-agent information:

_/opt/digitalocean/bin/do-agent --version_:
```
boa:~# /opt/digitalocean/bin/do-agent --version

do-agent (DigitalOcean Agent)

Version: 3.5.5
Revision: 8fe5d9e
Build Date: Thu Aug 1 17:36:22 UTC 2019
Go Version: go1.12.1
Website: https://github.com/digitalocean/do-agent

Copyright (c) 2019 DigitalOcean, Inc. All rights reserved.

This work is licensed under the terms of the Apache 2.0 license.
For a copy, see .
```

_apt-cache policy do-agent_:

```
boa:~# apt-cache policy do-agent
do-agent:
Installed: 3.5.5
Candidate: 3.5.5
Version table:
*** 3.5.5 500
500 http://repos.insights.digitalocean.com/apt/do-agent main/main amd64 Packages
100 /var/lib/dpkg/status
3.5.4 500
500 http://repos.insights.digitalocean.com/apt/do-agent main/main amd64 Packages
3.5.2 500
500 http://repos.insights.digitalocean.com/apt/do-agent main/main amd64 Packages
3.5.1 500
500 http://repos.insights.digitalocean.com/apt/do-agent main/main amd64 Packages
3.3.1 500
500 http://repos.insights.digitalocean.com/apt/do-agent main/main amd64 Packages
3.2.1 500
500 http://repos.insights.digitalocean.com/apt/do-agent main/main amd64 Packages
3.0.5 500
500 http://repos.insights.digitalocean.com/apt/do-agent main/main amd64 Packages
2.2.4 500
500 http://repos.insights.digitalocean.com/apt/do-agent main/main amd64 Packages
2.2.3 500
500 http://repos.insights.digitalocean.com/apt/do-agent main/main amd64 Packages
2.2.1 500
500 http://repos.insights.digitalocean.com/apt/do-agent main/main amd64 Packages
2.2.0 500
500 http://repos.insights.digitalocean.com/apt/do-agent main/main amd64 Packages
2.1.3 500
500 http://repos.insights.digitalocean.com/apt/do-agent main/main amd64 Packages
2.0.2 500
500 http://repos.insights.digitalocean.com/apt/do-agent main/main amd64 Packages
2.0.1 500
500 http://repos.insights.digitalocean.com/apt/do-agent main/main amd64 Packages
2.0.0 500
500 http://repos.insights.digitalocean.com/apt/do-agent main/main amd64 Packages
1.1.3 500
500 http://repos.insights.digitalocean.com/apt/do-agent main/main amd64 Packages
```