To test bootable flag in a cinder volume

This commit adds to cinder client a bootable capability
supported flags are  True or False.
And a testcase for changing the bootable flag from true to false.

Update bootable volume is not supported on icehouse
Added new flag under config.py - to enbale this feature by request
Change-Id: Ia23cab169c46d631444399ab1edd93c43cfb7ef5
diff --git a/etc/tempest.conf.sample b/etc/tempest.conf.sample
index 4418b9d..d7dd5e1 100644
--- a/etc/tempest.conf.sample
+++ b/etc/tempest.conf.sample
@@ -1169,3 +1169,7 @@
 
 # Is the v2 volume API enabled (boolean value)
 #api_v2 = true
+
+# Update bootable status of a volume Not implemented on icehouse
+# (boolean value)
+#bootable = false