Modified the current ``glance-manage db purge`` command to eliminate images
table from purging the records. Added new command
``glance-manage db purge_images_table`` to purge the records from images
table.
DocImpact
SecurityImpact
Change-Id: Ie6641659b54543ed9f96c393d664e52a26bfaf6a
Implements: blueprint mitigate-ossn-0075
(cherry picked from commit 5cc9d99935)