deployment: make scap proxies configured in one place

* moved misc::deployment::scap_proxy to deployment::scap_proxy
* made dsh/group/scap-proxies configurable via a class variable
* use the same variable in mediawiki::common to determine if the server
  is a scap proxy

This way we can finally change things in one place only.

Change-Id: I22dd6d44a4574f679451a606fab7750e7345bb6c
Signed-off-by: Giuseppe Lavagetto <glavagetto@wikimedia.org>