settings.py: Add a monitoring period setting

#132

buildgrid/settings.py

+# Copyright (C) 2018 Bloomberg LP
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#  <http://www.apache.org/licenses/LICENSE-2.0>
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+
 import hashlib
 
 
-# The hash function that CAS uses
+# Hash function used for computing digests:
 HASH = hashlib.sha256
+
+# Lenght in bytes of a hash string returned by HASH:
 HASH_LENGTH = HASH().digest_size * 2
+
+# Period, in seconds, for the monitoring cycle:
+MONITORING_PERIOD = 5.0