python: Add utility function to override config parameters

Add a utility method, SimObject.apply_config that can be used to
implement SimObject param overrides from the command line. This
function provides safe and convenient semantics for CLI assignment:

* The override expression is evaluated in a restricted environment. The
  only global variables are the child objects and params from the root
  object.

* Only params can be overridden. For example, calling methods or setting
  attributes on SimObjects isn't possible.

* Vectors use non-standard list semantics which enable something similar
  to glob expansion on the shell. For example, setting:

      root.system.cpu[0:2].numThreads = 2

  will override numThreads for cpu 0 and 1 and:

      root.system.cpus[0,2].numThreads = 2

  sets it for cpus 0 and 2.

The intention is that the helper method is called to override default
values before calling m5.instantiate.

Change-Id: I73f99da21d6d8ce1ff2ec8db2bb34338456f6799
Reviewed-on: https://gem5-review.googlesource.com/c/12984
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>

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