[Extension] Extension will not run if WALA use customerized configuration file path

Now WALA support to customerize confituration file path by useing parameter "-configuration-path"
If we use a customerized configuration file path to WALA, extension VMAccessForLinux will not successfully install.Because inside VMAccessForLinux extension,it will only check the default configuration file path(/etc/waagent.conf).If /etc/waagent.conf not exist,it will unistall.