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).

Author: rouilj

CHANGES.txt

@@ -131,6 +131,10 @@ Fixed:
 - 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
 

roundup/configuration.py

@@ -27,6 +27,8 @@
 import roundup.anypy.random_ as random_
 import binascii
 
+from roundup.backends import list_backends
+
 # XXX i don't think this module needs string translation, does it?
 
 ### Exceptions
@@ -336,6 +338,17 @@ def str2value(self, value):
         else:
             raise OptionValueError(self, value, self.class_description)
 
+class DatabaseBackend(Option):
+    """handle exact text of backend and make sure it's available"""
+    class_description = "Available backends: %s"%", ".join(list_backends())
+
+    def str2value(self, value):
+        _val = value.lower()
+        if _val in list_backends():
+            return _val
+        else:
+            raise OptionValueError(self, value, self.class_description)
+
 class HtmlToTextOption(Option):
 
     """What module should be used to convert emails with only text/html parts into text for display in roundup. Choose from beautifulsoup 4, dehtml - the internal code or none to disable html to text conversion. If beautifulsoup chosen but not available, dehtml will be used."""
@@ -894,11 +907,11 @@ def str2value(self, value):
             "is run, so it must be explicitly set to a non-empty string.\n"),
     )),
     ("rdbms", (
+        (DatabaseBackend, 'backend', NODEFAULT,
+            "Database backend."),
         (Option, 'name', 'roundup',
             "Name of the database to use.",
             ['MYSQL_DBNAME']),
-        (Option, 'backend', '',
-            "Database backend."),
         (NullableOption, 'host', 'localhost',
             "Database server host.",
             ['MYSQL_DBHOST']),
@@ -947,8 +960,8 @@ def str2value(self, value):
             "Database isolation level, currently supported for\n"
             "PostgreSQL and mysql. See, e.g.,\n"
             "http://www.postgresql.org/docs/9.1/static/transaction-iso.html"),
-    ), "Settings in this section are used"
-        " by RDBMS backends only"
+    ), "Settings in this section (except for backend) are used"
+        " by RDBMS backends only."
     ),
     ("logging", (
         (FilePathOption, "config", "",