Add new shelve compute feature flag

Shelving/unshelving is not supported across all drivers (e.g., Ironic).
This adds a new compute feature flag and skips tests where this function
is performed.

Change-Id: Ib94b479db2b691e73c6121c17cb47ae1c25636a8
diff --git a/etc/tempest.conf.sample b/etc/tempest.conf.sample
index 8a7ad9c..c3e2c5f 100644
--- a/etc/tempest.conf.sample
+++ b/etc/tempest.conf.sample
@@ -409,6 +409,10 @@
 # Does the test environment support pausing? (boolean value)
 #pause=true
 
+# Does the test environment support shelving/unshelving?
+# (boolean value)
+#shelve=true
+
 # Does the test environment support suspend/resume? (boolean
 # value)
 #suspend=true