Spec Lite: Introduce db sync check feature

problem: It is very hard for automation of deploy and upgrade operations to know if there are db migrations pending. It requires the automation to know what the latest version is, and compare that to the output of a command to check the current version, then interpret the potential difference somehow.
solution: Similar to the linked feature added to Keystone's manage command, Glance should support an operation which enumerates any outstanding db upgrade operations and provide user friendly message based on that status. Each expand, migrate, and contract operation required to upgrade the db should be listed in the proper order of execution in the response. This may be implemented by using a glance-manage db check option. When this option is present no db upgrades would be performed but potential operations would be reported, acting similar to the pattern of a dry-run.
impacts: Introduces new option to the db sync operation in glance-manage.
timeline: Queens RC-1
link: https://bugs.launchpad.net/keystone/+bug/1642212
reviewers: rosmaita, abhishekk, jokke
assignee: bhagyashris