Update git submodules

* Update charm-swift-storage from branch 'master'
  to ca420eccbb9e1cd51379b27ac684ef6e94d380e7
  - Enable swift payload upgrades for wallaby+
    
    Sync charm-helpers to pick up fix for openstack_upgrade_available
    that enables successful payload upgrades. This will need backporting
    to stable/wallaby.
    
    openstack-charmers/zaza#619
    Closes-Bug: #2040606
    Change-Id: Ib1586a486a7a01023bd59bfa70016481ad5f89dc