unit tests fixes for workdir api change

notable fixes are because of use of setDefaultWorkdir in unit.util.steps.BuildStepMixin, which sets workdir to 'wkdir'

ShellCommand was supporting setDefaultWorkdir, and MasterShellCommandm or ShellSequence were not.
Now all are supporting setDefaultWorkdir in the same way

Signed-off-by: Pierre Tardy <pierre.tardy@intel.com>
add relnote

Signed-off-by: Pierre Tardy <pierre.tardy@intel.com>