Add growuserdata feature

Since we're placing user data in a dedicated device which is itself set as a LVM PV, it is easy to extend and resize this volume lively with a shell command that we can execute through an SSH connection between the `denver` application and the D3nver instance.

Would be nice to have such feature available through the `denver` app so that we can easily expand the user data volume when a user is running out of space in his D3nver instance.