Fix workload parameter processing when a key is mapped to a JSON file

[gkamat]

Description

When the argument passed to the workload-params option includes a key-value pair, where the value is a JSON file, there parser may treat the entire option string incorrectly as a filename.

Issues Resolved

#502

Testing

Unit and integ tests. Added unit tests.

---

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[IanHoang]

Left one comment but looks good to me

[IanHoang]

Nit: We can use WorkloadConfigError from exceptions.py in place of the generic Exception class to be more descriptive.

# from exceptions.py
class WorkloadConfigError(BenchmarkError):
    """
    Thrown when something is wrong with the workload config e.g. user supplied a workload-param
    that can't be set
    """

[gkamat]

Nit: We can use WorkloadConfigError from exceptions.py in place of the generic Exception class to be more descriptive.

# from exceptions.py
class WorkloadConfigError(BenchmarkError):
    """
    Thrown when something is wrong with the workload config e.g. user supplied a workload-param
    that can't be set
    """

The reason Exception was used here is to catch all possible errors that might be raised by read_workload. For, instance, it might raise a WorkloadSyntaxError. However, it's correct that a wrapped WorkloadConfigError could be raised once the exception is caught. Will consider doing that in a future revision.