MINOR: server: refactor cli_find_server()

MINOR: server: refactor cli_find_server()

Refactor cli_find_server() by splitting its execution into two separated
functions. The first one parse_be_srv() is able to extract a backend and
server name from an argument of the form <bename>/<svname>. The second
function find_be_srv() is responsible to perform the lookup of a server
using a backend and server name.

Now cli_find_server() consists mainly of these two calls. The objective
is to be able to perform only the parsing or lookup parts, without
automatic CLI error display. This will be used to extend CLI "wait"
command handler.

CLI parser "delete server" has been updated to use the new
parse_be_srv() as an example of usage. In the future, it could be useful
to update all similar handlers relying on the same argument format.