BUG/MINOR: acme: can't override the default resolver

BUG/MINOR: acme: can't override the default resolver

httpclient_acme_init() was called in cfg_parse_acme() which is at
section parsing. httpclient_acme_init() also calls
httpclient_create_proxy() which could create a "default" resolvers
section if it doesn't exists.

If one tries to override the default resolvers section after an ACME
section, the resolvers section parsing will fail because the section was
already created by httpclient_create_proxy().

This patch fixes the issue by moving the initialization of the ACME
proxy to a pre_check callback, which is called just before
check_config_validity().

Must be backported in 3.2.