lxc-execute: allow lxc-init to log only when we have a valid log level

Right now if we use lxc-execute without log level set, we get error:
lxc: invalid log priority NOTSET.
Because we set log level manually in execute_start(), but didn't
check if we have a valid log level or not, so fix it.

Signed-off-by: Qiang Huang <h.huangqiang@huawei.com>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>