Add 'runat' parameter to command output interfaces

Add a new 'runat' parameter sos_get_command_output() to specify
a directory for the child to switch to before executing the given
command and propagate this through the various command output
collection interfaces.

Signed-off-by: Bryn M. Reeves <bmr@redhat.com>
Make OpenShift plugin use the runat parameter

OpenShift uses 'cd /path && cmd' in a call to add_cmd_output().
Replace this with the runat paramter and avoid shell syntax in
the plugin.

Related: Issue #253.

Signed-off-by: Bryn M. Reeves <bmr@redhat.com>