Allow configuration of a default value for a label.

This change allows a project admin or owner to define a default value
(or score) for a label. The default value is set in the project configurations
file with a 'defaultValue' key.  The defaultValue must be within the range of
valid label values.  It is an optional label setting, if not defined the
defaultValue for the label will be 0.  When a defaultValue is defined, that
value will get set in the Reply dialog by default.  A defaultValue can be set
to a score that is outside of the permissible range for a user.  In that case
the score that will get set in the Reply box will be the next closes score
to the defaultValue.

feature: Issue 2041
Change-Id: I12dece29b043e09eaab62cdcf56146a1380041bc

gerrit-extension-api/src/main/java/com/google/gerrit/extensions/common/LabelInfo.java

@@ -25,6 +25,7 @@ public class LabelInfo {
   public List<ApprovalInfo> all;
   public Map<String, String> values;
   public Short value;
+  public Short defaultValue;
   public Boolean optional;
   public Boolean blocking;
 }