Xhprof improvements

- Install xhprof unconditionally with php5. HHVM ships with it
  and it's a great development tool. Use the package since it
  Just Works and remove the chain of exec{} calls
- Refactor xhprof role into xhprofgui role which installs the
  xhprof_html from Github and aliases it to /xhprof as before.

Change-Id: Icac3c46e8838633c7bf52f54ebc51cc72ff9b020