Resolve "Add all graph attributes to the workflow description"

Closes #38 (closed)

Add the following keys to the workflow description: keywords, input_schema, execute_arguments and worker_options.

In addition GET /workflows and GET /workflows/descriptions now have an optional argument keywords which can be used to filter.

input_schema is to create a parameter form for execution (would be very useful in ewoksweb as well).

execute_arguments and worker_options are inputs for POST /execute/{identifier}. Note: maybe ewoksserver should merge the ones from the workflow with the ones provided too POST /execute/{identifier}? In this case the client does not have to (but can) provide execution and worker options. In that case, do these still need to be part of the workflow description?