New ipamodule_base_vars

There are common parameters in all modules like ipaadmin_principal and
ipaadmin_password. As this list of common parameters will be extended
soon, there is a need to reduce the code and documentation duplicates.

A ModuleDocFragment is added to provide the module documentation for the
common parameters. This is used in the modules with
extends_documentation_fragment.

ansible_freeipa_module has additional ipamodule_base_spec and
get_ipamodule_base_vars. ipamodule_base_spec extends argument_spec in
the module and get_ipamodule_base_vars is used to return a dict
containing the common parameters.