Fix problem where image data is not read from a pipe.

For image-updae and image-create commands, glanceclient attempts to
determine whether image data should be uploaded based on the presence
of data on stdin. Unforunately it is difficult to determine if data is
available, especially when standard in is from a pipe.

This is especially problematic for update operations, where data must
only be uploaded if the image is in queued state. For example data may
be uploaded when the user only wants to rename an image, but the rename
will be rejected because data cannot be uploaded to an unqueued image.

This patch removes the check that attempts to determine if data is
available to read as it didn't work for pipes. It also re-introduces a
check for image state in the update operation, so that glanceclient only
attempts to read data if the image being updated is in queued state.

The image state check is part of the original patchset that was removed
so the patchset could have a single focus [1]

This patch also removes a test for handling empty stdin, and adds a test
for reading stdin from a pipe.

[1] https://review.openstack.org/#/c/27536/3/glanceclient/v1/shell.py

Fixes: bug 1184566
Related to: bug 1173044

Change-Id: I8d37f6412a0bf9ca21cbd75cde6a4d5a174e5545