Support -I option in gnt-instance modify
Support -I option in gnt-instance modify

When converting an instance from plain to DRBD, a secondary
node has to be chosen. Allow using an IAllocator to do this
choice.

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

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