MINOR: proxy: simplify parsing 'backend/server'

MINOR: proxy: simplify parsing 'backend/server'

Several CLI handlers use a server argument specified with the format
'<backend>/<server>'. The parsing of this arguement is done in two
steps, first splitting the string with '/' delimiter and then use
get_backend_server() to retrieve the server instance.

Refactor this code sections with the following changes :
* splitting is reimplented using ist API
* get_backend_server() is removed. Instead use the already existing
  proxy_be_by_name() then server_find_by_name() which contains
  duplicated code with the now removed function.

No functional change occurs with this commit. However, it will be useful
to add new configuration options reusing the same '<backend>/<server>'
for reverse connect.