Increase maximum URI size for EC2 API to 16k

The EC2 API supports both HTTP GET and POST agnostically.  It also supports
user-data of 16k -- meaning that client tools could generate 16k URIs.
The WSGI default limit is 8k; this raises it.

Fixes bug 1098646.

Change-Id: Idec460d88b2affab970c9d9f39fa61295db035c5
Merge "Increase maximum URI size for EC2 API to 16k"