acme-common: fallback to deprecated use_staging if the staging option is not set

acme-common: fallback to deprecated use_staging if the staging option is not set

The use_staging option was deprecated in 9d2d8787cafdfb49339766a00ced422fa75cb883.
But it still has a bigger priority than the staging option.
This happens because config_get_bool returns 0 when the use_staging option wasn't set.
So the next check for the staging var emptiness is always false.

As the simplest fix, use the config_get staging that returns a plain string when the option is not set and if it's empty then fallback to the use_staging.

Once the use_staging option is removed we should get back to the config_get_bool staging.

Also use config_get_bool debug.

Signed-off-by: Sergey Ponomarev <stokito@gmail.com>