DateTime inputs are not handled correctly

[torse]

First problem is a TypeError in the wps-agent during parsing of "datetime" input parameters. The problematic line seems to be [1] where strptime is used to parse the xs:dateTime/ISO8601 string into a python object. This can be fixed by using strftime.

Second problem is that the datetime object is not formatted for the command line argument. This results in a command parameter like "--start_time 2017-01-01 00:00:00" as space between date and time is the default python output. However the underlying process.py except a ISO8601 string and thus fails to parse the parameter. Thus the output for the commandline needs to be formatted as well.

[1] https://github.com/geoserver/wps-remote/blob/master/src/wpsremote/computation_job_input.py#L45