move read_proxy_conf from PVE::API2Tools to new Utils module

move read_proxy_conf from PVE::API2Tools to new Utils module

move the read_proxy_conf method into a new perl module
'PVE::APIServer::Utils'.
It now takes the proxy_name (e.g. pveproxy, pmgproxy) as variable to be used
for the configfile location (/etc/default/$proxy_name)

This serves as preparation to make pmgproxy configurable in the same way as
pveproxy.

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>