Allow operators to customize max header size

HTTP messages max header line size has been increased from 8K to 16K
by default to allow using tokens including up to 14-15 catalog
entries.  The same issue
(https://bugs.launchpad.net/swift/+bug/1190149) may occur again in
the future, if keystone's catalog grows further.

Allowing operators to customize the max header size, will allow them
to have Cinder working whatever the size of the catalog (if the option
is properly set).

Change-Id: Iac269d96fadc96c1e9383dc03422f2d87196dd9d
Related-Bug: #1190149
DocImpact
Merge "Allow operators to customize max header size"