Fixed issues in miq_ae_field_spec

1. Use destory_all when deleting fields instead of deleting 1 at a time
2. Use ae_fields.clear to remove bad entries from ae_fields
3. When adding same field names add them after the first one has been
validated

These changes were done to make the GIT based model to be in sync with
AR model.
@Fryguy @jrafanie 
In this scenario all three would have worked. The main goal was to remove a invalid (unsaved) record from the association. For the GIT version when I do a save on the parent, if the bad one is not removed from the association I will thrown an Invalid Record, even if the good one is being saved.