Create config file for diagnostic snapshot generation

* create config file on the master node
* pass path to config file as a parameter
* renamed nailgun_dump to shotgun
* fixed test and fixed wrong rspec usage
  (removed before/after hacks for mocking)

Depends on:
* patch in fuel-web for shotgun and nailgun
  https://review.openstack.org/89260

Closes-bug: #1310244
Change-Id: If70badfb90afc16541c79ac99255fc22d54aecf4
Remove hardcoded path to shotgun

Change-Id: I624efa4a67c584bacf9ec104095f6faa1dffd0a9
Related-bug: #1309741