Fix encoding for setInstanceStatus
Fix encoding for setInstanceStatus

The corresponding optional values in python are encoded in
the poor-man's-Maybe way, i.e., null for Nothing and unmodified
for Just.

Signed-off-by: Klaus Aehlig <4ce51de36d9068cd7db2eec1151e0f3f95419008@google.com>
Reviewed-by: Helga Velroyen <2d0126b1ec6f9ec5cbec4a7012aa14faeaef10f5@google.com>

onponomarev/ganeti,mbakke/ganeti,leshchevds/ganeti,mbakke/ganeti,andir/ganeti,ganeti/ganeti,leshchevds/ganeti,mbakke/ganeti,leshchevds/ganeti,bitemyapp/ganeti,andir/ganeti,yiannist/ganeti,yiannist/ganeti,grnet/snf-ganeti,ganeti/ganeti,onponomarev/ganeti,ganeti/ganeti,bitemyapp/ganeti,grnet/snf-ganeti,andir/ganeti,yiannist/ganeti