Add an alternate default boto file for bootstrapping source.

download_from_google_storage.py

Index: download_from_google_storage.py
===================================================================
--- download_from_google_storage.py	(revision 207434)
+++ download_from_google_storage.py	(working copy)
@@ -46,6 +46,12 @@
     env = os.environ.copy()
     if self.boto_path:
       env['AWS_CREDENTIAL_FILE'] = self.boto_path
+    else:
+      custompath = env.get('AWS_CREDENTIAL_FILE', '~/.boto') + '.depot_tools'
+      custompath = os.path.expanduser(custompath)
+      if os.path.exists(custompath):
+        env['AWS_CREDENTIAL_FILE'] = custompath
+
     return subprocess2.call((sys.executable, self.path) + args,
                             env=env,
                             timeout=self.timeout)