Prevent write_config from corrupting container config

write_config doesn't check the value sig_name function returns,
this causes write_config to produce corrupted container config when
using non-predefined signal names.

Signed-off-by: Alexander Vladimirov <alexander.idkfa.vladimirov@gmail.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>