shellsequence: implement step

this steps permits one to launch multiple shell
commands in a single step. Each shell command
has its own optional stdio log, and haltOnFailure, etc parameter.
They however, all share the same workdir, use-pty, env parameters.

the skipping of commands can be driven by overriding
one method of that class (see doc).

Signed-off-by: Ion Alberdi <ion.alberdi@intel.com>