Refactoring of diagnostic snapshot generation logic

Nailgun:
* removed snapshot generation calls from nailgun
* removed endpoint nailgun_dump from nailgun egg

Shotgun:
* added endpoint 'shotgun'
* added command line interface, now
  it accepts --config key
* created nailgun independent logger

Depends on:
* patch in astute https://review.openstack.org/89261

Closes-bug: #1310244
Change-Id: Ibcf4129efc1b21a91d0c27ea72f0a31ca9891e99
Merge "Refactoring of diagnostic snapshot generation logic"