dns-challenge: add 'use-proxy' property

dns-challenge: add 'use-proxy' property

this patch adds an optional 'use-proxy' property to the dns
challenges.

If set to 1 the caller is expected to add the proxy url in the plugin
config, which is then set as 'http_proxy' and 'https_proxy'
environment variable by the plugin caller (and then used by curl)

Tested with the pdns plugin, direct traffic to the pdns server being
dropped, and a configured squid proxy

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>