Supports disabling auto_trace_sampling for ESP installed in GCE vm without docker

[qiwzhang]

There is a flag added --disable_cloud_trace_auto_sampling in start_esp. If ESP is installed in GCE vm without docker, it is not easy to set this flag or any other start_esp flags.

Two options:

1. read a metadata field, and pass its value to the start_esp flag

2. use env variable, this code https://github.com/cloudendpoints/endpoints-tools/blob/master/start_esp/nginx-init.d/nginx#L35 will use it. The env variable can be set in
   /etc/default/nginx in the GCE.

Let us do 2)