Add console_output compute feature flag

Not all hypervisors support getting serial console logs.  This adds
a new compute feature flag.  It skips tests that stress this call and
avoids logging console logs to debug if it is not supported.

Change-Id: Icc37e9f3497fb7bd72f359197663c71abbf16921
diff --git a/etc/tempest.conf.sample b/etc/tempest.conf.sample
index 96ef11a..75ab72e 100644
--- a/etc/tempest.conf.sample
+++ b/etc/tempest.conf.sample
@@ -405,6 +405,10 @@
 # password? (boolean value)
 #change_password=false
 
+# Does the test environment support obtaining instance serial
+# console output? (boolean value)
+#console_output=true
+
 # Does the test environment support resizing? (boolean value)
 #resize=false