Client/Tools/Action: add shell attribute

Add an option to specify whether a command should be executed within
a shell to enable flow control and other shell-specific syntax. The behavior
of 1.3.1 before had changed from 1.2.x. It was documented that the Action
always is executed within a shell. But the restructuring with
Bcfg2.Utils.Executor change the default mode to execute all Actions without
a shell. This patch adds an option, so that everyone could specify the default
mode with the Rules or per Action.