Document 2.5 to 2.6 upgrade path for templates

[miguelsorianod]

Write documentation to upgrade from 2.5 to 2.6 release in templates.

For the moment the following should be documented:

• Migrate all DCs to use the new ServiceAccounts and migrate to be able to use RedHat registry Prepare usage of registry.redhat.io registry #124

• Update existing ImageStream images

• How to move DBs DCs to ImageStreams, also including the modification of the "postgresql" imagestream to use the tag formatting that we have in all the other ones Migrate dbs to imagestreams #122 and Rename postgresql imagestream #155

• Maybe some PostgreSQL template addition related change? Add system's PostgreSQL compatibility #129 --> No specific changes needed for upgrade

• Remove WildcardRouter Remove wildcard router from operator and templates #135

• Add/Modify new Zync-related changes Zync routing #145 . What is observed as template changes related to this commit is the addition of the new elements and removal of default system routes (system-master, system-provider, system-developer, apicast-production, apicast-staging). The addition of new elements should be documented in the upgrade procedure but the removal of old routes probably not because the user might want to keep them

• Redis Enterprise Support Configurable system redis urls #114 related changes

• Redis Sentinel Support Configurable redis sentinels in system #137 related changes

• Apply Redis Sentinel environment variable fixes in backend-cron and backend-worker Add sentinel env fix #133