ipauser: Add support to modify GECOS field.

This patch adds a new parameter to ipauser, 'gecos', which can be used
to set the 'gecos' field of an IPA user. The default behavior of
automatically set the GECOS field to "<first> <last>" is not modified,
it is only possible to change the field to a custom value.

No validation on the value provided is done, as it is with FreeIPA.