It looks like all of the VPN options are stored in /etc/NetworkManager/VPN
SSTP is in there, but doesn't appear as an option in the Network Manager GUI (Gnome and Unity)user@computer:/etc/NetworkManager/VPN$ ls -1
nm-openconnect-service.name
nm-openvpn-service.name
nm-pptp-service.name
nm-sstp-service.name
nm-vpnc-service.name
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