Implement sanity check for malformed pi.cfg

[fredreichbier]

Appliance versions < 3.0 have the issue already mentioned in #60: If the pi-appliance tool reads and writes /etc/privacyidea/pi.cfg, some of the resulting values are malformed, e.g.:

import logging
PI_PEPPER = 'YKroh9ZXchPh2DUVRexdnrnr'
SQLALCHEMY_DATABASE_URI = 'mysql://pi:0MY3mjw6Tw8R@localhost/pi'
PI_AUDIT_SQL_TRUNCATE = 'True'
PI_ENCFILE = '/etc/privacyidea/enckey'
PI_AUDIT_KEY_PUBLIC = '/etc/privacyidea/public.pem'
PI_LOGLEVEL = logging.INFO
PI_LOGFILE = '/var/log/privacyidea/privacyidea.log'
PI_ENGINE_REGISTRY_CLASS = '"shared"'
PI_AUDIT_POOL_SIZE = '20'
PI_AUDIT_KEY_PRIVATE = '/etc/privacyidea/private.pem'
SECRET_KEY = 'tR2HbnXb7Tk8C1cwxTWE8yEG'
SUPERUSER_REALM = ['super']

Note that privacyIDEA still works because the malformed values are handled as follows:

• PI_AUDIT_SQL_TRUNCATE = 'True' is treated like PI_AUDIT_SQL_TRUNCATE = True because we check for the config option using .get("PI_AUDIT_SQL_TRUNCATE"): https://github.com/privacyidea/privacyidea/blob/5cd6730e3925931740e0ea72c051140f17c57aff/privacyidea/lib/auditmodules/sqlaudit.py#L225
• PI_ENGINE_REGISTRY_CLASS = '"shared": '"shared'" is an invalid value, so we fall back to "null", which disables connection pooling for SQL audit and resolver connections.
• PI_AUDIT_POOL_SIZE = '20' causes privacyIDEA to throw an exception here:
  https://github.com/privacyidea/privacyidea/blob/5cd6730e3925931740e0ea72c051140f17c57aff/privacyidea/lib/auditmodules/sqlaudit.py#L125
  but this exception is caught. As a result, pooling of SQL audit connections is disabled.

Currently, the user has to edit the config file manually to restore the correct config values.

In the version of the pi-appliance tool, we could add a sanity check that detects the malformed pi.cfg above, and offers to fix the values automatically.