[autotest] Update some afe_utils methods to use host._afe_host.

Updated the following methods to use local host._afe_host.labels:
- get_labels
- get_board
- get_os
- get_build

Also updated callers as well since it used to return a list of Label
objects, now it returns a list of strings.

BUG=None
TEST=unittests and tryjob on wolf-paladin with hwtests.

Change-Id: Ib1ab3b4fea5d69d2e587a0d09ef0f3463915e11e
Reviewed-on: https://chromium-review.googlesource.com/360512
Commit-Ready: Justin Giorgi <jgiorgi@google.com>
Tested-by: Kevin Cheng <kevcheng@chromium.org>
Reviewed-by: Simran Basi <sbasi@chromium.org>
diff --git a/server/cros/dynamic_suite/constants.py b/server/cros/dynamic_suite/constants.py
index 7f750b3..044e27c 100644
--- a/server/cros/dynamic_suite/constants.py
+++ b/server/cros/dynamic_suite/constants.py
@@ -24,6 +24,7 @@
 JOB_REPO_URL = 'job_repo_url'
 VERSION_PREFIX = 'cros-version:'
 BOARD_PREFIX = 'board:'
+OS_PREFIX = 'os'
 
 # Bug filing
 ISSUE_OPEN = 'open'