taxonomy_allowed_values should use entity_label

Currently when using the Entity Translation and Title modules to translate taxonomy term names, the term name is not being translated when used in an options form widget for the taxonomy reference field, see #1557708: Support for taxonomy_options_list. This can be fixed by using entity_label instead of $term->name in taxonomy_allowed_values. This also helps term name translation when using i18n_taxonomy (part of i18n). i18n_taxonomy currently does some voodoo to change the options_list_callback for taxonomy_term_reference fields. It works but would not be needed anymore with this patch.

Since using entity_label is the correct behavior anyway, I think it would be the cleanest solution to change the Taxonomy module instead of making the Title module do some more voodoo. Maybe using the correct label as returned by entity_label would do some good in other places as well, like taxonomy_autocomplete_validate for example.