_translate_from_glance() can cause an unnecessary HTTP request

After returning from a get() call to python-glanceclient, nova 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.

Change-Id: I67b7dd16a94fe60d873c012f6bd246ab24500d5a
Closes-Bug: #1275173
Merge "_translate_from_glance() can cause an unnecessary HTTP request"