rdt: partition level defaults in config

Currently, we require that allocation is explicitly specified on the partition level. A more user friendly way would probably be to assume 100% if nothing is specified, especially because many usage scenarios seem to only involve one partition, in which case it seems dump to require to explicitly specify 100% allocations for all resources.

This might be lead to surprises/slightly misleading error messages when multiple partitions are specified: Cache allocations on partition level are exclusive and cannot overlap. Basically with multiple partitions an error occurs if any partition requests 100% of cache lines. But probably that can be mitigated by improving error messages.

An example:
```
partitions:
default:
classes:
...
```

would imply
```
partitions:
default:
l2Allocation: "100%"
l3Allocation: "100%"
mbAllocation: ["100%", 4294967295]
classes:
...
```