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."
Allow to configure the dev-type openvpn should use

This comes in handy if one would like to explicitly name the interface
openvpn should create. This now can be accomplished by setting 'dev' to
some name and 'dev_type' to either tun or tap.
Merge remote-tracking branch 'origin/feature/allow-multiple-dns-servers-to-be-pushed' into develop

* origin/feature/allow-multiple-dns-servers-to-be-pushed:
  Allow to supply an array of dns servers to be pushed via dhcp-option
Merge remote-tracking branch 'origin/feature/add-option-dev-type' into develop

* origin/feature/add-option-dev-type:
  Allow to configure the dev-type openvpn should use