New parameter for swift backend

New parameter is 'swift_store_large_object_size'.

Sometimes default value (5G) doesn't work when user attempts to
make a snapshots with size > 300 Mb. So it may be usefull to have
a possibity to adjust this parameter to your needs.

Change-Id: I08336b91b0cd294a86326605fb6c28653e8eedb8
Merge "New parameter for swift backend"