rpcd-mod-lxc: add lxc-create ubus bindings

Example call:

$ ubus call lxc create '{ "name": "bar", "template": "download", "args": [ "--no-validate", "--dist", "debian", "--release", "sid", "--arch", "i386" ] }'

Signed-off-by: Luka Perkov <luka@openwrt.org>