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.
The changes here are done to enable simpler switching between the AR version and the File/GIT based version.  It's easier to keep this method name for now than introduce the chaining Arel methods.