ansible_freeipa_module: Cleanup of unused functions and attributes

Moved ipamodule_base_spec into IPAAnsibleModule.
This simplifies to add addtitional base vars to derived classes without
the need to dusplicae the code to extend argument_spec.

Removed execute_api_command.
This is not used anymore.

Removed get_ipamodule_base_vars.
This is not used anymore.

Removed self.ipaadmin_* and self.ccache_* from IPAAnsibleModule.__init__
These are not used and therefore can be removed.