support Oracle Java, if installed

fdroidserver should prefer OpenJDK, but if the user only has Oracle Java installed, then fdroidserver should use it. Here's the stacktrace from an install with only Oracle Java:

fdroidadmin@ubuntufdroidserver:/usr/share/nginx/html/fdroid$ fdroid init
Unknown exception found!
Traceback (most recent call last):
  File "/usr/bin/fdroid", line 110, in <module>
    main()
  File "/usr/bin/fdroid", line 91, in main
    mod.main()
  File "/usr/lib/python2.7/dist-packages/fdroidserver/init.py", line 126, in main
    test_config = common.get_default_config()
  File "/usr/lib/python2.7/dist-packages/fdroidserver/common.py", line 46, in get_default_config
    'keytool': glob.glob('/usr/lib/jvm/java-1.7.0-openjdk-*/bin/keytool')[0],
IndexError: list index out of range