Doesn't accept pkcs12 files

[Ralf Hildebrandt]

While openvpn gladly accepts pkcs12 files:
pkcs12 client.p12

the network-manager-openvpn still insists on seperate files for ca, cert, and key, e.g. like:

ca ca.crt
cert client.crt
key client.key

Please make the use of PKCS12 files possible. That way the users only have 66.66% less things to configure (and less errors to make).