[autotest] Rename site_host.SiteHost to cros_host.CrosHost

Rename site_host.SiteHost to cros_host.CrosHost.
cros_host.CrosHost is the new chromeos specific host class.
The concept 'SiteHost' is deprecated as we are dealing with
multiple types of hosts (e.g. ServoHost).
The concept of a single'SiteHost' doesn't meet our needs any more.

All references are updated.

SerialHost is now subclassing RemoteHost and mixes in CrosHost
in hosts.factory.create_host().

BUG=chromium:273833
TEST=Trybot build passes; Repair/Verify pass;
Dummy suite passes;
Server side test platform_InstallTestImage
which is affected by this CL passes.
DEPLOY=scheduler

Change-Id: I5e59194996c67c6b5d7fdcaf40d594ea08de1af5
Reviewed-on: https://chromium-review.googlesource.com/167182
Tested-by: Fang Deng <fdeng@chromium.org>
Reviewed-by: Simran Basi <sbasi@chromium.org>
Commit-Queue: Fang Deng <fdeng@chromium.org>
diff --git a/server/hosts/remote.py b/server/hosts/remote.py
index d1b4b46..e2ddba5 100644
--- a/server/hosts/remote.py
+++ b/server/hosts/remote.py
@@ -1,5 +1,4 @@
-"""This class defines the Remote host class, mixing in the SiteHost class
-if it is available."""
+"""This class defines the Remote host class."""
 
 import os, logging, urllib
 from autotest_lib.client.common_lib import error