[Autotest][Py3] Update server/hosts/__init__ and host_attributes to have full imports

Updating relative imports to full imports in support of python3 migration

BUG=chromium:990593
TEST=dummy_Pass (python2 and 3)

Change-Id: Ic920023b2d6d02b69af85a08957a06c60fd63c22
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/autotest/+/2589560
Reviewed-by: Gregory Nisbet <gregorynisbet@google.com>
Commit-Queue: Derek Beckett <dbeckett@chromium.org>
Tested-by: Derek Beckett <dbeckett@chromium.org>
diff --git a/server/host_attributes.py b/server/host_attributes.py
index 326855e..bfaafa5 100644
--- a/server/host_attributes.py
+++ b/server/host_attributes.py
@@ -2,8 +2,13 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-import utils
-
+try:
+    from autotest_lib.server import utils
+except ImportError:
+    import logging
+    logging.warning("Could not import utils from autotest_lib.server. "
+                    "Reverting to simply: `import utils`.")
+    import utils
 
 class base_host_attributes(object):
     def __init__(self, host):
diff --git a/server/hosts/__init__.py b/server/hosts/__init__.py
index d8bd4dc..e58e28d 100644
--- a/server/hosts/__init__.py
+++ b/server/hosts/__init__.py
@@ -7,18 +7,33 @@
 You should 'import hosts' instead of importing every available host module.
 """
 
+try:
+    # host abstract classes
+    from autotest_lib.server.hosts.base_classes import Host
+    from autotest_lib.server.hosts.remote import RemoteHost
 
-# host abstract classes
-from base_classes import Host
-from remote import RemoteHost
+    # host implementation classes
+    from autotest_lib.server.hosts.ssh_host import SSHHost
+    from autotest_lib.server.hosts.cros_host import CrosHost
+    from autotest_lib.server.hosts.chameleon_host import ChameleonHost
+    from autotest_lib.server.hosts.servo_host import ServoHost
+    from autotest_lib.server.hosts.labstation_host import LabstationHost
 
-# host implementation classes
-from ssh_host import SSHHost
-from cros_host import CrosHost
-from chameleon_host import ChameleonHost
-from servo_host import ServoHost
-from labstation_host import LabstationHost
+    # factory function
+    from autotest_lib.server.hosts.factory import create_host
+    from autotest_lib.server.hosts.factory import create_target_machine
+except ImportError:
+    # host abstract classes
+    from base_classes import Host
+    from remote import RemoteHost
 
-# factory function
-from factory import create_host
-from factory import create_target_machine
+    # host implementation classes
+    from ssh_host import SSHHost
+    from cros_host import CrosHost
+    from chameleon_host import ChameleonHost
+    from servo_host import ServoHost
+    from labstation_host import LabstationHost
+
+    # factory function
+    from factory import create_host
+    from factory import create_target_machine