Better logic for auto-detecting pull_threads

On a single-CPU vm, we ended up with 0 pull_threads, which resulted in
the pull process getting stuck.

Signed-off-by: Konstantin Ryabitsev <konstantin@linuxfoundation.org>
diff --git a/grokmirror/pull.py b/grokmirror/pull.py
index 276089d..cfc03e3 100755
--- a/grokmirror/pull.py
+++ b/grokmirror/pull.py
@@ -1163,9 +1163,11 @@
     lastrun = 0
 
     pull_threads = config['pull'].getint('pull_threads', 0)
-    if pull_threads < 1:
+    if pull_threads < 1 and mp.cpu_count() > 1:
         # take half of available CPUs by default
         pull_threads = int(mp.cpu_count() / 2)
+    else:
+        pull_threads = 1
 
     busy = set()
     done = list()