MINOR: server: improve parsing error for server-template

When parsing a server configuration line, a global context is set to the
current server instance. This allows to automatically prefix any
warning/error messages by the server name. This prefix is generated via
generate_usermsgs_ctx_str().

The output was not useful for server-template lines as server <id> is
NULL for them. Thus, this patch improves generate_usermsgs_ctx_str() to
identify a server-template. Now, a dedicated 'server-template' prefix is
generated, with <tmpl_info.prefix> used instead of NULL <id> field.

* Example of a previous error message :
[ALERT]    (39934) : config : [./hap.cfg:54] : 'server-template be/(null)' : error detected while parsing [...]
* Example of a new error message :
[ALERT]    (40205) : config : [./hap.cfg:54] : 'server-template be/local' : error detected while parsing [...]