Fix issue2551029 (Jinja2 template install error) by deleting config.ini from all templates. Looks like classic and minimal never included a config.ini. Presence of config.ini stopped 'roundup-admin install' from generating an up to date config.ini. Created special option class for backend database. Comment now uses roundup.backend.list_backends to give user indication of valid values. Also we now get an error if user specifies invalid backend. Moved backend setting to top or [rdbms] section. It is the most important thing in there and if set to anydbm (or some other non-rdbm) the rest of the settings can be ignored. Change rdbms config section comment to indicate that the backend setting was used by other database backends. (e.g. if set to anydbm).

--- a/CHANGES.txt	Thu May 30 20:15:16 2019 -0400
+++ b/CHANGES.txt	Sat Jun 01 09:05:23 2019 -0400
@@ -131,6 +131,10 @@
 - issue2551033: prevent reverse engineering hidden data by using etags
   as an oracle to identify when the right data has been
   guessed. (Joseph Myers, John Rouillard)
+- issue2551029: Jinja2 template install error. Update configuration
+  code to make sure valid backend database is set. Remove config.ini
+  from templates to make sure that roundup-admin install writes a new
+  default config.ini based on configuration.py.
 
 2018-07-13 1.6.0