change default value for s3_store_host

Changing the default value for s3_store_host parameter from
127.0.0.1:8080 to s3.amazonaws.com in glance-api.conf and
glance-cache.conf.

DocImpact: The following parameter is changed in glance-api.conf
and glance-cache.conf.
-s3_store_host = 127.0.0.1:8080
+s3_store_host = s3.amazonaws.com

Change-Id: I39f5bca280b83853021f379c1b7ebe29cff182b9