Accessing the shell

Public key for target nodes defined in Settings tab; doc'ed
in User Guide section.

New section in Operations Guide covers:
- 0100-public-keys -- how to manually upload a Public Key
- 0200-ssh-fuel-master -- how to ssh into Fuel Master
- 0300-ssh-target-nodes -- how to ssh into target nodes

The XFS and two Galera sections were erroneously nested
under "Introduction" so I had to make those top-level headers
to avoid having them nested under this material.

Change-Id: Idc457ac705be5a1e505e3721efbddd331d4bd973
Closes-Bug: 1375825
Merge "Accessing the shell"