Respect the thread_limit setting included in the previous commit

Due to timing, I couldn't cleanly include this in the previous commit,
but here it is.
Now Broker actions, including checkins will reference this new setting
when determining how many threads it should use.

broker/broker.py

@@ -24,6 +24,7 @@
 from broker import exceptions, helpers
 from broker.hosts import Host
 from broker.providers import PROVIDER_ACTIONS, PROVIDERS, _provider_imports
+from broker.settings import settings
 
 # load all the provider class so they are registered
 for _import in _provider_imports:
@@ -86,7 +87,8 @@ def _act(self, provider, method, checkout=False):
         method_obj = getattr(provider_inst, method)
         logger.debug(f"On {provider_inst=} executing {method_obj=} with params {self._kwargs=}.")
         # Overkill for a single action, cleaner than splitting the logic
-        with ThreadPoolExecutor() as workers:
+        max_workers = min(count, int(settings.thread_limit)) if settings.thread_limit else None
+        with ThreadPoolExecutor(max_workers=max_workers) as workers:
             tasks = [workers.submit(method_obj, **self._kwargs) for _ in range(count)]
             result = []
             for task in as_completed(tasks):
@@ -202,8 +204,8 @@ def checkin(self, sequential=False, host=None, in_context=False):
         if not hosts:
             logger.debug("Checkin called with no hosts, taking no action")
             return
-
-        with ThreadPoolExecutor(max_workers=1 if sequential else None) as workers:
+        max_workers = min(len(hosts), int(settings.thread_limit)) if settings.thread_limit else None
+        with ThreadPoolExecutor(max_workers=1 if sequential else max_workers) as workers:
             completed_checkins = as_completed(
                 # reversing over a copy of the list to avoid skipping
                 workers.submit(self._checkin, _host)