Improve consistency of pluggable module import paths

[fredreichbier]

privacyIDEA has several modules which are "pluggable", i.e. their concrete implementation class is imported from a module specified in the config file. Examples are the audit module, HSM module, or monitoring module. While working on #1335, I noticed that we have slight inconsistencies there.

• Audit module and monitoring module: The PI_AUDIT_CLASS and PI_MONITORING_MODULE options contain a path to a Python module which contains the concrete implementation class which has a fixed name:

PI_MONITORING_MODULE = "privacyidea.lib.monitoringmodule.sqlstats" # the class is privacyidea.lib.monitoringmodule.sqlstats.Monitoring
PI_AUDIT_MODULE = ""privacyidea.lib.auditmodules.sqlaudit" # the class is privacyidea.lib.monitoringmodule.sqlstats.Audit

• HSM module, SMS provider, PIN handler policy: The PI_HSM_MODULE option contains the path to the concrete implementation class:

PI_HSM_MODULE = "privacyidea.lib.security.default.DefaultSecurityModule"

The SMS provider and PIN handler classes are not configured via pi.cfg, but they also contain the path to the concrete implementation class, e.g. "privacyidea.lib.smsprovider.HttpSMSProvider.HttpSMSProvider"

• Engine registry class: This is not actually pluggable, the only valid values for PI_ENGINE_REGISTRY_CLASS are "null" and "shared". But it could be made pluggable!

As everything is documented properly, I don't think we have to do something about that right now, but I wanted to open this issue to keep this in mind if we rework our plugin interface in the future.