_translate_from_glance() can cause an unnecessary HTTP request

After returning from a get() call to python-glanceclient, cinder runs a
translation function on the returned Image to get the data it wants. Part of
this process is checking for an expected set of attributes, one of which is
the deletion time ('deleted_at'). However, if the image has not been deleted,
deleted_at key will not exist. This forces another call to glance to occur for
the same image. A similar problem exists for the checksum attribute, which does
not exist before an image is active. The fix here is to only consider
deleted_at and checksum if they are expected to be present.

This change was made in nova as change I67b7dd16

Change-Id: Iedc16cb9316f9610fdb8ac03f448bc375a4e6bfa
Closes-Bug: #1275173
(cherry picked from commit da13c6285bb0aee55cfbc93f55ce2e2b7d6a28f2)