Update git submodules

Update git submodules

* Update docs/submodules/snaps from branch 'master'
  - Merge "Refactored multi-part images."
  - Refactored multi-part images.

    The logic used for creating multi-part images for the Danube release
    was copied all throughout the tests, which was not sustainable.
    This patch encapsulates the kernel and ramdisk images within the
    same OpenStackImage object as the disk image.

    JIRA: SNAPS-59

    Change-Id: I45ec0ccc666accb4e768c41e249e6130dc07f6d1
    Signed-off-by: spisarski <s.pisarski@cablelabs.com>