Parse the arguments of an execution and make it available inside Rundeck object

Hi,

Here is an improvement of the API in order to retrieve the optional execution parameters and store them in a map. I'm using this feature inside the Jenkins rundeck plugin, in order to expose the execution parameters as environment variable (I'll make a pull request using 2.1-SNAPSHOT very soon).

Could you consider merging my change if it suits you?

Thanks,
Reynald