Support L2TP over IPsec VPN Connections

The VPN configuration only supports connecting to PPTP and OpenVPN servers.

While OpenVPN works great, it's sometimes not supported - for instance, in enterprise settings. My work uses L2TP over IPsec which works on OS X but is not an option in Elementary OS (as of 0.4 Loki).

Launchpad Details: [#LP1624770](https://bugs.launchpad.net/bugs/1624770) Mark Dain - 2016-09-17 22:56:18 +0000

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