Signingscript

Added
- Added new dep, `mar`. Python now requires lzma support; CentOS signing scriptworkers will need the xz-devel package.
- Added a new kwarg `raise_on_empty_list` to `get_suitable_signing_servers`. When True, and if the list of suitable signing servers is empty, the function raises a `SigningScriptError`.
- Added `sign_mar384_with_autograph_hash`. This is new functionality to only send the hash of the mar to autograph, to reduce autograph and network load.
- Added the `autograph_hash_only_mar384` signing format

Changed
- Updated travis docker image to `mozilla/autograph:2.3.2`

Added
- Added optional `datadog_port` and `datadog_host` to the config file

Changed
- Read the datadog config from the config file

Changed
- Updated signing server host cert to one with the new mac signing server alt names AND has a new subject (so it doesn't get ignored)

Changed
- Updated signing server host cert to one with the new mac signing server alt names

Fixed
- Initialize a hardcoded datadog statsd port of 8135 to avoid conflicting with collectd.

Changed
- Configuration: `taskcluster_scope_prefix` now becomes `taskcluster_scope_prefixes` and takes a JSON array.