hydra: add Coverity Scan plugin

This adds a Hydra plugin for users to submit their open source projects
to the Coverity Scan system for analysis.

First, add a <coverityscan> section to your Hydra config, including the
access token, project name, and email, and a regex specifying jobs to
upload:

    <coverityscan>
      project = testrix
      jobs    = foobar:.*:coverity.*
      email   = aseipp@pobox.com
      token   = ${builtins.readFile ./coverity-token}
    </coverityscan>

This will upload the scan results for any job whose name matches
'coverity.*' in any jobset in the Hydra 'foobar' project, for the
Coverity Scan project named 'testrix'.

Note that one upload will occur per job matched by the regular
expression - so be careful with how many builds you upload.

The jobs which are matched by the jobs specification must have a file in
their output path of the form:

  $out/tarballs/...-cov-int.(xz|lzma|zip|bz2|tgz)

The file must have the 'cov-int' directory produced by `cov-build` in
the root.

(You can also output something into
$out/nix-support/hydra-build-products for the Hydra UI.)

This file will be found in the store, and uploaded to the service
directly using your access credentials. Note the exact extension: don't
use .tar.xz, only use .xz specifically.

Signed-off-by: Austin Seipp <aseipp@pobox.com>