(PUP-2660) user: set the user should-value when generating ssh_authorized_keys

When purging authorized SSH keys, the user type generates ssh_authorized_keys
resources using eval_generate. The ensure property receives a should value of
"absent" so the keys are removed. However, those resources are non-functional
because the user property lacks a should value. The resource fails during
Provider::Ssh_authorized_keys::Parsed#flush.

Fixed by setting the appropriate user value for each generated key.
Merge remote-tracking branch 'ffrank/ticket/3.6.1/PUP-2660-keys-not-purged-missing-user' into stable

* ffrank/ticket/3.6.1/PUP-2660-keys-not-purged-missing-user:
  (PUP-2660) user: set the user should-value when generating ssh_authorized_keys
(PUP-2659) Synchronize all webrick requests.

Puppet had some scattered thread safety code, which was removed in
4da350b, because Puppet itself is not using threads.  However, Webrick
starts a new thread to handle each incoming request.  This was probably
causing subtle problems during simultaneous requests to a webrick
master, but since 3.6.1 the issue became much more noticeable due to the
fix for PUP-2610 merged in 0dc5ad3 which was fixing a case where rack
masters would lose track of newly established environment loaders using
the master settings.  The change to Puppet::Context used a new
mark/rollback method for overriding context making it very obvious when
concurrent threads would collide attempting to rollback the same marker
and exhaust the context stack.

We are fixing this by synchronizing around incoming webrick requests,
effectively serializing their access to Puppet's innards.  Initial
manual benchmarking did not show a significant decrease in performance,
probably because Ruby uses green threads handled by the vm which cannot
take advantage of multiple processors.
(PUP-2659) Add a concurrency acceptance test

It is difficult to test concurrent requests with Beaker as it does not
allow parrallel requests to be made.  I'm using backgrounded subprocess
in a shell script to get around this, and I'm only making direct catalog
requests via curl, since an agent will halt if it spots another's pidfile in
existence on the same node.  Still, this is one of the major calls that
the agent makes, and it does effectively expose PUP-2659 on a
non-synchronized webrick master.
(PUP-2639) Change default timeout to 3 minutes

The 5 second timeout was selected assuming that the major use case would
be around a development workflow in which a user edits manifests on a
master and then immediately runs an agent to see the effect. It turns
out that this caused a lot of problems for a much more common and
impactful case of users updating to directory environments where they
ended up with excessive CPU usage. Nearly any value here is arbitrary
except 0 (purely development workflow) and unlimited (best choice for
production systems).

This makes the almost arbitrary decision that a 3 minute timeout will
create less confusion for upgrading users. It means that a development
workflow will require more restarts, however. :(
Merge pull request #2710 from zaphod42/feature/stable/pup-2639-increase-environment-timeout

(PUP-2639) Change default timeout to 3 minutes
Merge pull request #2708 from jpartlow/issue/stable/pup-2659-synchronize-webrick-rest

Issue/stable/pup 2659 synchronize webrick rest