Add /var/cache/swift to the list of directories that must be owned by swift.

/var/cache/swift must be writeable by the user swift. python-swift in ubuntu
grizzly packages does not create /var/cache/swift with the proper permissions.
This was fixed and released for havana packages.

Related-Bug: #1234657
Change-Id: I27e4835b4d1833c493347cfcc51f4b9489c29d63