Allow contributor agreements to be optional

By default the contributor agreements are disabled, as there are
no agreements in the database.  Admins which require agreements
must enable them in system_config.

This permits friendly community installations, such as a corporate
location, where access automatically implies agreement to contribute
because it is work-for-hire.

Signed-off-by: Shawn O. Pearce <sop@google.com>

appjar/src/main/java/com/google/gerrit/server/GerritServer.java

@@ -278,6 +278,7 @@ public class GerritServer {
     final GerritConfig r = new GerritConfig();
     r.setCanonicalUrl(getCanonicalURL());
     r.setSshdPort(sConfig.sshdPort);
+    r.setUseContributorAgreements(sConfig.useContributorAgreements);
     if (sConfig.gitwebUrl != null) {
       r.setGitwebLink(new GitwebLink(sConfig.gitwebUrl));
     }

appjar/src/main/java/com/google/gerrit/server/ssh/Receive.java

@@ -89,7 +89,9 @@ class Receive extends AbstractGitCommand {
   @Override
   protected void runImpl() throws IOException, Failure {
     server = getGerritServer();
-    verifyActiveContributorAgreement();
+    if (Common.getGerritConfig().isUseContributorAgreements()) {
+      verifyActiveContributorAgreement();
+    }
     loadMyEmails();
     lookup(reviewerId, "reviewer", reviewerEmail);
     lookup(ccId, "cc", ccEmail);