Add support for CentOS/RHEL to haproxy-marathon-bridge

This patch simply adds checks inside the install_haproxy_system function for what OS is being used, using lsb_release. If the OS is not CentOS, RHEL, Ubuntu, or Debian - exit 1 with an error message.
Looking back at the script, I was mistaken. The indents are not what I'm accustomed to in Bash scripts, but do follow a pattern.

Typically I see lines such as these:
```    if [[ -f /etc/init/haproxy.conf ]]
    then reload haproxy ## In case it switches to Upstart
    else /etc/init.d/haproxy reload
    fi
```
Written like this:
```
if [[ -f /etc/init/haproxy.conf ]]; then
  reload haproxy ## In case it switches to Upstart
else
  /etc/init.d/haproxy reload
fi
```
I've left those as-is and applied your suggestion. I also replaced haproxy.cfg with a variable.