Outdated contract of APIs accepting job parameters as String

[fmbenhassine]

JobOperator#start and MetaDataInstanceFactory#createJobExecution expect job parameters to be passed as a single string which is either a comma separated or a new line separated key/value pairs.

The comma separated format is incompatible with the new format introduced in v5 (see #3960) . PropertiesConverter (which is the utility used behind the scene by those APIs) should be updated to only support new line delimited key/value pairs.