Documentation updates

Our admin guide was very very trivial. This patch updates it to contain
advice on when to use which commands, removes the instance
administration part from the installation guide (moved to the admin
guide), and adds a walkthrough document that should be useable as a
starting point for new admins.

The patch also adds emacs variables to the documents, and rewraps some
which were not already at 72 chars.

The doc updates also show backwards-compatible commands for Ganeti 2.0,
as we don't have a good up-to-date 2.0 document and people might refer
to this set of documentation even when running that.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>