Add docstrings for utils.host

Cleaning up a little technical debt by adding docstrings
for our public APIs.

Note: I plan to submit a patch per file as part of this bug work.
These patches won't depend on each other. The last patch of
the series will close the bug.

Change-Id: Ib848349ae8f7497aa9f2646e8db20c26aa4f3d8c
Partial-Bug: #1614594

neutron_lib/utils/host.py

@@ -15,6 +15,11 @@ import multiprocessing
 
 
 def cpu_count():
+    """Get the system CPU count.
+
+    :returns: The number of CPUs on the system as an int. If there's an issue
+    fetching the CPU count on the system, a default value of 1 is returned.
+    """
     try:
         return multiprocessing.cpu_count()
     except NotImplementedError: