media: disable OCS fallback when on SfB server
SfB clients no longer send initial call INVITE in multipart/alternative
format with ms-proxy-2007fallback section meant as a compatibility mode
with old OCS clients, and at least the client for Windows Phone replies
with 488/Not Acceptable Here upon receiving such invitation from Sipe.
Since we can be reasonably confident we won't ever encounter ancient
clients in SfB environment, let's disable the fallback mode if we detect
our server is Skype for Business.