Check memory size before setting it

Check memory size before setting it

With this change when a user asks for a new memory size for an instance,
the number is checked instead of just applied. The operation fails only
if the instance would not be able to restart on its primary node, but
generates warnings should it be impossible to failover the instance or
should the computation be impossible due to nodes being unreachable.

This is a forward-port from branches/ganeti-1.2

Original-Reviewed-by: iustinp
Reviewed-by: iustinp