serial: use serial_load from REANA Commons
serial: use serial_load from REANA Commons

* Serial load from REANA Commons includes parameter interpolation
  so variable described as `$varname` and `${varname}` are replaced
  by the values set in the reana.yaml inputs.parameters section. If
  something goes wrong with spec validation or parameter interpolation
  an exception is thrown
  (closes reanahub/reana-workflow-engine-serial#34).

reanahub/reana-client,reanahub/reana-client