Allow users to customize max header size

Also increases the default value from 8192 to 16384

The former max header (8192) is exceeded in the following scenario
- Auth tokens built with a keystone v3 API catalog
- A catalog with approximately 8 or more endpoints defined

Equivalent fixes have been proposed in other projects

Change-Id: Ic714166440e2aa08b619ab1e5e5e0cf3cea3e309
Closes-Bug: 1190149
Merge "Allow users to customize max header size"