GDPR cookie sync configuration

[dbemiller]

This issue is for adding GDPR support to the Prebid Server config. For an overview of GDPR support in Prebid Server, see #501

Requirements

Prebid Server host companies must be able to:

1. Identify themselves as one of the members of the global vendors list
2. Define how Prebid Server should behave if the Request doesn't carry any GDPR info in it.

Working design

We plan to add the following config options:

gdpr:
  host_vendor_id: 23
  usersync_if_ambiguous: true

If these are undefined, we will assume that the publisher doesn't care about GDPR and continue to sync users all the time.

If defined:

• host_vendor_id must be an integer > 0 which identifies the Prebid Server host company in the Global Vendors List.
• usersync_if_ambiguous will determine whether or not usersyncs are done if the incoming request has no gdpr info.

usersync_if_ambiguous is optional, and will be treated as false if host_vendor_id is defined.

See #504 for changes to the core logic in the endpoints.

[bretg]

LGTM