This change allows the role to accept either lxc2 or lxc3 config
keys, plus a list of key substitutions to make when writing the
lxc config file.
This allows a set of config keys to remain defined as variables
outside this role and generate a valid config on both lxc2 and lxc3
based hosts.
Change-Id: Ifc871a9fcaf77ff36cfcc1c87b1f406862d46d22