Create a new Ganeti/Types.hs module

There are already three cases where we copied type definitions between
the htools-specific types into the main ganeti code. Let's stop doing
this ☺️

 and create a common types module that holds these.

Note that there already exists BasicTypes.hs, but that refers to very
low-level types, and can't use TH derivation itself.

A side effect of this unification is that there is a small conflict
between AdminStatus/AdminOffline and InstanceStatus/AdminOffline. As
such, I renamed AdminOffline and AdminDown to StatusOffline/StatusDown
in the InstanceStatus type.

The patch also moves the tests related to these types to a new test
module.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Adeodato Simo <dato@google.com>