Allow to supply an array of dns servers to be pushed via dhcp-option

From openvpn's man page:
"--dhcp-option type [parm]
 (...)
  DNS addr -- Set primary domain name server address.  Repeat this
    option to set secondary DNS server addresses."
Merge pull request #7 from mmz-srf/feature/allow-multiple-dns-servers-to-be-pushed

Allow to supply an array of dns servers to be pushed via dhcp-option