lxc-ubuntu: Add extra options

This adds support for "packages", "user" and "password"

Signed-off-by: Guilhem Lettron <guilhem.lettron@optiflows.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>
lxc-ubuntu: Move package install to post-process

Signed-off-by: Guilhem Lettron <guilhem.lettron@optiflows.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>
lxc-ubuntu: Add mirror and security-mirror options

Signed-off-by: Guilhem Lettron <guilhem.lettron@optiflows.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>
lxc-ubuntu: Factorize @LOCALSTATEDIR@ variable

Signed-off-by: Guilhem Lettron <guilhem.lettron@optiflows.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>