scheduler: merge Reserve and Unreserve plugins

Previously, separate interfaces were defined for Reserve and Unreserve
plugins. However, in nearly all cases, a plugin that allocates a
resource using Reserve will likely want to register itself for Unreserve
as well in order to free the allocated resource at the end of a failed
scheduling/binding cycle. Having separate plugins for Reserve and
Unreserve also adds unnecessary config toil. To that end, this patch
aims to merge the two plugins into a single interface called a
ReservePlugin that requires implementing both the Reserve and Unreserve
methods.

pkg/scheduler/testing/framework_helpers.go

@@ -100,8 +100,6 @@ func getPluginSetByExtension(plugins *schedulerapi.Plugins, extension string) *s
 		return initializeIfNeeded(&plugins.Bind)
 	case "Reserve":
 		return initializeIfNeeded(&plugins.Reserve)
-	case "Unreserve":
-		return initializeIfNeeded(&plugins.Unreserve)
 	case "Permit":
 		return initializeIfNeeded(&plugins.Permit)
 	case "PreBind":