Added new evidence helper methods

[jleaniz]

This pull request accomplishes two things:

1. Adds two new methods to the evidence module to generate a list of attributes for the Evidence sub-classes that can be processed by Turbinia as evidence types (e.g. rawdisk). It will be used by the Turbinia API client command-line tool to dynamically build a list of command line arguments for each evidence type.

2. Removes an unnecessary level of inheritance for DiskPartition and GoogleCloudDisk

3. Adds missing REQUIRED_ATTRIBUTES to some evidence types.

Fixes #1139

[jleaniz]

Note to self: It is not entirely clear why DiskPartition and GoogleCloudDisk inherit from RawDisk as they override the _preprocess and _postprocess methods which is the only thing that could be of benefit through inheritance in this case.

[jleaniz]

it looks like that level of inheritance was not needed so i removed it because it makes it more difficult to apply the REQUIRED_ATTRIBUTES since constructor arguments differ for RawDisk, DiskPartition and GoogleCloudDisk.