Remove "image_cache_invalid_entry_grace_period" option

 "image_cache_invalid_entry_grace_period" option is unused
in *.py, but the option is used in glance-cache.conf.

 I removed "image_cache_invalid_entry_grace_period" option
from glance-cache.conf.

Change-Id: Id235945f9cc89219118e4a3ec077f0e887a3b3ef
Closes-Bug: #1249949
Merge "Remove "image_cache_invalid_entry_grace_period" option"