Unify platform_InstallTestImage with servo repair.

The body of platform_InstallTestImage was an almost verbatim copy
of SiteHost._servo_repair().  Replaced that body with a call to the
host method.

BUG=None
TEST=run_remote_tests platform_InstallTestImage

Change-Id: Ie2dc1fb2adcba544cd472f4f1052a440e21b988e
Reviewed-on: https://gerrit.chromium.org/gerrit/47389
Commit-Queue: Richard Barnette <jrbarnette@chromium.org>
Reviewed-by: Richard Barnette <jrbarnette@chromium.org>
Tested-by: Richard Barnette <jrbarnette@chromium.org>
diff --git a/global_config.ini b/global_config.ini
index d8f560e..16b11bd 100644
--- a/global_config.ini
+++ b/global_config.ini
@@ -156,6 +156,7 @@
 image_url_pattern: %s/update/%s
 log_url_pattern: http://%s/tko/retrieve_logs.cgi?job=/results/%s/
 package_url_pattern: %s/static/archive/%s/autotest/packages
+servo_url_pattern: %s/static/servo-images/%s_test_image.bin
 
 # Username and password for connecting to remote power control switches of
 # the "Sentry Switched CDU" type