enables use of jinja < 2.6

Commit 2b51a6f0c39b4df56823afe9d114dab886e3359e introduced options for gitfs_remotes in a pillar by using a jinja test to see if a parameter is a mapping (dict etc.). This feature however is only available in jinja 2.6 or newer (see http://jinja.pocoo.org/docs/dev/templates/#mapping).
Although this version of Jinja is available on Ubuntu, other OS / package managers do provide older versions (2.2.1 in RedHat 6).

This change makes use of the "iterable" test which should do the exact same thing.