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
GlusterFS: Fix create/restore backup

Allow create_backup for GlusterFS volumes, but
only when snapshots do not exist.  (For now.)

Restore is a no-op for the driver, so allow it.

Related-Bug: 1247743
Closes-Bug: 1275977

Change-Id: I50b8f6cac684c967c7374bb43247a396ce936157
Move create_, remove_ and ensure_export from drivers

create_export, remove_export, ensure_export are almost the same in
LVM and BlockDevice drivers. So they should be moved to
a common cinder.volume.iscsi module.

Change-Id: Id7aa0e10d6b346356312416aee179a1fe8bacef1
Ceph backup driver tidyup

Fixed typos and cleaned up log messages and docstrings.

Closes-Bug: 1276969
Change-Id: Idb1ba4f3315edb76fefc993b38b0590b4794345b
Changes to cast variable as integer as XML API fails.

Code change to ensure that the type of the size variable is correct
when cinder is invoked through XML API. The extend volume code path
differs from volume create where the flow portion verifies that the
size variable is an integer; for extend the earliest place to cast
the size variable is in volume_actions.py.

Change-Id: Iaaed463f988769b8093604cd3b677b270dd2c5cd
Closes-bug: 1277233
Merge "Ceph backup driver tidyup"
Merge "Move create_, remove_ and ensure_export from drivers"
Merge "Changes to cast variable as integer as XML API fails."
Merge "Allow operators to customize max header size"
Merge "GlusterFS: Fix create/restore backup"