maven_jar: Support download of artifacts from Maven snapshot repo

When the repository type is MAVEN_SNAPSHOT, the artifact is downloaded
from the Maven snapshot repository [1].

The artifact ID is given in the form:

 group:name:version:snapshot

where `group`, `name` and `version` are as before, and `snapshot` is
the snapshot timestamp (i.e. '20160407.201829-175').

[1] https://oss.sonatype.org/content/repositories/snapshots/

Change-Id: I992eb1461aa976ccc5137669270abe704f260500
diff --git a/tools/util.py b/tools/util.py
index 96f6047..08a803f 100644
--- a/tools/util.py
+++ b/tools/util.py
@@ -20,6 +20,7 @@
   'GERRIT_API': 'https://gerrit-api.commondatastorage.googleapis.com/release',
   'MAVEN_CENTRAL': 'http://repo1.maven.org/maven2',
   'MAVEN_LOCAL': 'file://' + path.expanduser('~/.m2/repository'),
+  'MAVEN_SNAPSHOT': 'https://oss.sonatype.org/content/repositories/snapshots',
 }