Split the hooks env building in two parts

This patch moves some of the environment processing from _BuildEnv to a
new _RunWrapper command which does the stringification and adds the
sstore variables.

The reasoning is that the sstore can be fresher than before the
execution (e.g.  in case of cluster init).

In order to support thise, we also need to modify cmdlib.LUInitCluster:
  - memorize the sstore and cfgw newly created in the Exec function
  - no need to build the custom environment in the BuildHooks