Update the local copy of the image schema to reflect Image API 2.6.

Change-Id: Ib0e56027927880d0fa198ffd8ea4b57e39f9d0fe
Closes-bug: #1762044
Depends-on: https://review.openstack.org/#/c/559501/

Add checks to image-import command so that it provides better user
feedback in failure situations.

Change-Id: I8b6b32c3d1d1a745aa68ff8dc629419dff9bb130
Closes-bug: #1758718

Add checks to the image-create-via-import commmand so that it provides
better user feedback and doesn't begin the import workflow unless the
input has a chance of succeeding.  Preserves backward compatibility
with the current image-create command by (1) allowing an image record
only to be created when no import-method is specified AND no data is
supplied, and (2) doing the glance-direct workflow when no import-
method is specified AND data is provided.  Also adds the ability for
the import-method to be set as an env var OS_IMAGE_IMPORT_METHOD.

Change-Id: I0a225f5471a9311217b5d90ebb5fd415c369129a
Closes-bug: #1758149

The do_image_download code has a check to make sure that there's
a place to put the data (either filename or stdout redirect) before
initiating the download.  The location of this check was moved by
change I841bebeda38814235079429eca0b1e5fd2f04dae to happen at the
beginning of the function.  The two intermittently failing tests
do not explicitly address the check condition, and as a result the
tests do exit early, but before they can check what they're supposed
to be testing.

Closes-bug: #1759951

Change-Id: I3c85bb358f669504b364d55618c21382b7a2a66b

Fail image-create-via-import requests for the web-download import
method that don't include values for container_format or disk_format.

Closes-bug: #1757927

Change-Id: Ic5c81916823ff32f2dbddd32b40e825de0697dc9