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/119014)
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 Nova working whatever the size of the catalog (if the option
is properly set).

Related-Bug: #1190149
DocImpact

Change-Id: I96694ac9bc242267cfc9f4d0c18b2b7fe0000460
Merge "Allow operators to customize max header size"