MINOR: server: Add 'server-template' new keyword supported in backend sections.

MINOR: server: Add 'server-template' new keyword supported in backend sections.

This patch makes backend sections support 'server-template' new keyword.
Such 'server-template' objects are parsed similarly to a 'server' object
by parse_server() function, but its first arguments are as follows:
    server-template <ID prefix> <nb | range> <ip | fqdn>:<port> ...

The remaining arguments are the same as for 'server' lines.

With such server template declarations, servers may be allocated with IDs
built from <ID prefix> and <nb | range> arguments.

For instance declaring:
    server-template foo 1-5 google.com:80 ...
or
    server-template foo 5 google.com:80 ...

would be equivalent to declare:
    server foo1 google.com:80 ...
    server foo2 google.com:80 ...
    server foo3 google.com:80 ...
    server foo4 google.com:80 ...
    server foo5 google.com:80 ...