--sign_tool is an optional flag applicable to virtApex.

For not virtApex, this results in a KeyError.
line 151, in <module>\n', '  File
"/usr/local/google/home/baligh/clients/goog/master/out/host/linux-x86/bin/sign_apex/sign_apex.py",
line 144, in main\n', "KeyError: 'sign_tool'\n"]

BUG: 193504286
Test: TH
Change-Id: Id982e5c57086ada78168163d2293813df121847d

tools/releasetools/sign_apex.py

@@ -141,7 +141,7 @@ def main(argv):
       signing_args=options.get('payload_extra_args'),
       codename_to_api_level_map=options.get(
           'codename_to_api_level_map', {}),
-      sign_tool=options['sign_tool'])
+      sign_tool=options.get('sign_tool', None))
   shutil.copyfile(signed_apex, args[1])
   logger.info("done.")