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 pull request #8 from mmz-srf/feature/add-option-dev-type

Allow to configure the dev-type openvpn should use