lxc-create: Require --template be passed

It's often been reported that the behavior of lxc-create without -t is a
bit confusing. This change makes lxc-create require the --template
option and introduces a new "none" special value which when set will
fallback to the old template-less behavior.

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>
Don't trigger SYSERROR for optional mounts

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>