Listofshellcommands

Implementation of a step that runs a list of shell commands. All the commands share the same
workdir, env or usePTY configuration. The tuning of how the steps are executed can be implemented
by inheriting from that class and overriding methods.

N.B: I did not manage to get rid of the MISSING BB REFERENCE: bb:step:ListOfShellCommands error
when generating the doc. I'll keep digging into that.