Prechecks do not validate custom configs

[alpha23]

Train, centos/source
kolla-ansible 2.9.3
ansible 2.8.5

Given that there are deprecated options between versions, custom config options from previous versions can cause issues with services. For example, from a previous version, I had included 'default_store = file' in a config/glance/glance-api.conf file which resulted in the upgrade to train "succeeding" (it did not fail) for glance, yet the glance-api container was stuck on 'restarting'. While one could theoretically review every project and every custom config to verify if there are any issues or conflicts, such an endeavor is quite time consuming. A precheck that verified this could have easily caught this option mis-specification and thus prevented excess downtime.