Configurable data set descriptors should support fixed parameter values

In the Report Descriptors and Data Set Descriptors that allow for specifying report definitions as yml configuration files in the "configuration" directory, there is currently no possibility to specify parameter values for data sets other than straight mappings down from the report definition.

This ticket should add the capability to either:

a) hard code specific parameter values for a given data set (this would allow re-using a single SQL file for multiple data sets and changing for different metadata)

b) specify a non-straight-through mapping (i.e. ${startDate+1m})

mogoodrich and ddesimone FYI