There is a workaround to get l2tp-ipsec on 15.04 but its not very elegant.
I uninstalled strongswan and installed an old openswan trusty .deb package.
I then found .deb files of the old l2tp-ipsec-vpn and l2tp-ipsec-vpn-daemon from trusty by Werner Jaeger.
I installed those and was able to connect. However, this took quite a bit of time to get working properly and required installing deprecated packages.
There is a workaround to get l2tp-ipsec on 15.04 but its not very elegant.
I uninstalled strongswan and installed an old openswan trusty .deb package.
I then found .deb files of the old l2tp-ipsec-vpn and l2tp-ipsec- vpn-daemon from trusty by Werner Jaeger.
I installed those and was able to connect. However, this took quite a bit of time to get working properly and required installing deprecated packages.