Merge 'EvacMode' and 'NodeEvacMode' which are associated with the
duplicated constants 'IALLOCATOR_NEVAC_*' and 'NODE_EVAC_*'.  This
merge moves the type 'EvacMode' to 'Ganeti.Types' in order to avoid a
circular dependency between 'Ganeti.HsConstants' and
'Ganeti.HTools.Types'.  The type 'EvacMode' is preferred to the type
'NodeEvacMode' given that its use is more widespread.

Signed-off-by: Jose A. Lopes <jabolopes@google.com>
Reviewed-by: Klaus Aehlig <aehlig@google.com>