Make bin/swift testable part 2

Follow up for patch https://review.openstack.org/#/c/76487/

Change-Id: I6801b75cb78a3da0631ca8a60b7c2756b8930e72
Use the standard library's copy of mock when it's available.

Change-Id: Id6a31157582efe47cc3a74e6658679c2bec14767
Update help message to specify unit of --segment-size option.

Change-Id: Ic3f9a733329c19c4e0bb85676da28a8d1d74232b
Closes-Bug: 1302566
Merge "Use the standard library's copy of mock when it's available."
Merge "Make bin/swift testable part 2"