Use 'GetInstanceLVsByNode' from config file instead of Instance's
MapLVsByNode method.

Also remove all_lvs computation from config's 'AddInstance' method. In
order to compute the lvs we have to use the 'GetInstanceLVsByNode'
method but the instance object doesn't exist in the config yet. In
addition, the computation of all_lvs will be obsolete once the disks
object will be separated from the instance object, since when calling
'AddInstance' (adding an instance to the config file), the instance will
not have attached any disks.

Signed-off-by: Ilias Tsitsimpis <iliastsi@grnet.gr>
Reviewed-by: Jose A. Lopes <jabolopes@google.com>