Set the user agent for all gcloud_apis requests.
Set the user agent for all gcloud_apis requests.

Change-Id: Ic5d3e6cbe085a7bf226cc966dbf33d2a9ab270cf

mbrukman/gcloud,skelterjohn/gcloud_apis,webmaven/gcloud,mbrukman/gcloud,skelterjohn/gcloud_apis,webmaven/gcloud