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.
I prefer the indents as well. I kept to the style in place within this script, which was relatively lacking in whitespace. I'd be happy to indent this block and repair the indenting of the rest of the file as well, if that would be ok.