Support downloading from other Maven repositories

Developers may now request buck to use a local mirror by setting
the URL into local.properties, a file already ignored under the
current buck build process, e.g.:

  download.GERRIT = http://nexus.my-company.com/
  download.MAVEN_CENTRAL = http://nexus.my-company.com/

To support this usage buck now passes to download_jar.py only the name
of the repository in the URL.  download_jar parses local.properties
(if present) and replaces the "MAVEN_CENTRAL:" prefix with the URL
supplied by the user.

Because the URL can vary cache entries in buck-cache are now using the
SHA-1 we expect/verify against, instead of the SHA-1 of the URL.  This
makes it easier to find a specific JAR in the cache.  The artifact,
version and SHA-1 as named in the maven_jar() rule are the strings
used in the file name in buck-cache.

If no SHA-1 verification happens (e.g.  source attachment JARs that
are not included into the build) then sha1(canonical_url) is used.
Here the canonical URL uses the repository id prefix string, e.g.:

  MAVEN_CENTRAL:org/apache/mina/mina-core/2.0.5/mina-core-2.0.5-sources.jar

Change-Id: I5a469ab15fd36b81bf76f6b51e1110dfdcf04c86
diff --git a/lib/maven.defs b/lib/maven.defs
index a179deb8..2d2d656 100644
--- a/lib/maven.defs
+++ b/lib/maven.defs
@@ -12,8 +12,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-GERRIT = 'http://gerrit-maven.commondatastorage.googleapis.com'
-MAVEN_CENTRAL = 'http://repo1.maven.org/maven2'
+GERRIT = 'GERRIT:'
+MAVEN_CENTRAL = 'MAVEN_CENTRAL:'
 
 def define_license(name):
   n = 'LICENSE-' + name