ipauser: Use date string, not datetime object for expiration dates

So far a datetime object was created for the expiration dates
krbpasswordexpiration and krbprincipalexpiration. This resulted in also
sending these objects to the API. With this change, the dates are
converted into strings using the LDAP_GENERALIZED_TIME_FORMAT defined in
ipalib.constants. This way only strings are used with the IPA API.

A new function has been added to ansible_freeipa_module:

- date_string: Convert datetime to gernalized time format string

This fuction is used on the result of user_show to convert the
expiration dates to the gernalized time format string.

The existing function date_format in ansible_freeipa_module has been
renamed to convert_date and fixed in the way that it also uses
date_string to return a gernalized time format string and not a
datetime object. This function was only used in the ipauser module so
far.