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.
For the conditional, if there are multiple statements for each block then I would prefer having the statements indented. This is the style used with other (long) conditional blocks in this script. For example:
```
if [[ $os == "CentOS" ]] || [[ $os == "RHEL" ]]
then
  sudo yum install -y haproxy
  sudo chkconfig haproxy on
elif [[ $os == "Ubuntu" ]] || [[ $os == "Debian" ]]
then
  sudo env DEBIAN_FRONTEND=noninteractive aptitude install -y haproxy
  sudo sed -i 's/^ENABLED=0/ENABLED=1/' /etc/default/haproxy
else
  echo "$os is not a supported OS for this feature."
  exit 1
fi