cfgupgrade: Add support for Ganeti 2.2

Since configuration changes are now usually done in the configuration
writer class, cfgupgrade's only task is to change the config version.
This means we can easily support upgrading from Ganeti 2.0 and 2.1 to
2.2.

This patch also brings a couple of small cleanups.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>