Remove path_prefix property from the runner and TES model

[giventocode]

Overview:

The node task definition has an outputs field that contains a list of output files or directories. If an output has a path_prefix property, the runner uses the value in the path as relative path pattern. The runner uploads the matching files to the storage system. The TES server maps this property to the path_prefix field in the TES task model (v 1.1). In addition, the TES server uses this functionality to upload log files after the task is completed.

However, this property is no longer part of the TES task spec in the latest proposal. Therefore, we need to remove it from the node task definition as well as the TES model on the server. This property makes the implementation of #364 more complex.

AC:

• The runner should assume that the path value is an absolute path pattern and upload the matching files if the output type is FILE or not set.
• Modify the TES server to use the new functionality instead of path_prefix when creating additional outputs.

[BMurri]

It appears that ga4gh/task-execution-schemas#194 isn't entirely settled. This should be deferred until the final resolution is determined.