Use "target" url in redfish standard instead of hardcode

[Lin Yang]

Current when resetting power state of the computer system, it generate URL with hard coded address '/Actions/ComputerSystem.Reset'.
https://github.com/openstack/sushy/blob/ec200bb3c00aa8caa1104d08560fd448b4bab37c/sushy/resources/system/system.py#L147

Actually redfish standard contains "target" URL for each action, should use it instead of hard code.

  "Actions" : {
    "#ComposedNode.Reset" : {
      "target" : "/redfish/v1/Nodes/2/Actions/ComputerSystem.Reset",
      "<email address hidden>" : [ "On", "ForceOff", "GracefulShutdown", "ForceRestart", "Nmi", "GracefulRestart", "ForceOn", "PushPowerButton" ]
    }
  }

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/440822

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/440822
Committed: https://git.openstack.org/cgit/openstack/sushy/commit/?id=0165d26672c7c3f22105231add37876119c62616
Submitter: Jenkins
Branch: master

commit 0165d26672c7c3f22105231add37876119c62616
Author: Lin Yang <email address hidden>
Date: Thu Mar 2 13:42:35 2017 -0800

    Fix hardcoded reset system path

    Previously, the path of resetting system is hardcoded to
    '/Actions/ComputerSystem.Reset'. Actually redfish provide 'target'
    attribution for each action. So use it instead of hardcode.

    Change-Id: Id5aaa418ff5ffc693afc593db5c4dc852b5b6c33
    Closes-Bug: #1668434