BUG/MINOR: server: set auto SNI for dynamic servers

BUG/MINOR: server: set auto SNI for dynamic servers

Auto SNI configuration is configured during check config validity.
However, nothing was implemented for dynamic servers.

Fix this by implementing auto SNI configuration during "add server" CLI
handler. Auto SNI configuration code is moved in a dedicated function
srv_configure_auto_sni() called both for static and dynamic servers.

Along with this, allows the keyword "no-sni-auto" on dynamic servers, so
that this process can be deactivated if wanted. Note that "sni-auto"
remains unavailable as it only makes sense with default-servers which
are never used for dynamic server creation.

This must be backported up to 3.3.