Adding qiime map file

This adds the creation of the qiime mapping file (combination of prep/sample template) at the when the prep template is added. Note that the code is ready for review, it tests pass, but we should merge https://github.com/biocore/qiita/pull/677 first. 
I think this can be done with [`load_template_to_dataframe`](https://github.com/biocore/qiita/blob/0803c6614cd436ce000e170070e0343a5f38cc35/qiita_db/metadata_template.py#L1640).