Handle nested xtrace states (Quantum edition)

The lib/quantum_* directories include plugin files that also
contains the $XTRACE bits at entry and exit.  The nested XTRACE handling overwrote
the value from lib/quantum.  So...make the nested files use their own
XTRACE variables.

Change-Id: Ib5e643371666b21402eef2ec58bfb1dfb7e1ccc4
Quantum/plugins/ryu: minor update for Quantum Ryu plugin

- updated package list for Ryu
  ryu plugin/agent needs python-netifaces
- quantum-ryu-agent also needs ryu module

Change-Id: I5b49efceb65e8139a49a8e82f55ea6aa7d1eebac
Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Create tools/install_prereqs.sh

* Factor system package prereq installs out to tools/install_prereqs.sh
* Set minimum time between runs with PREREQ_RERUN_HOURS
  default = 2 hours
* Create export_proxy_variables
* Force an update with install_prereqs.sh -f or by setting
  FORCE_PREREQ=true

Fixed an issue with exit/return in tools/install_prereqs.sh

Change-Id: I9a62090ad2f900b9b150cacb9cb02b326cb46972
Merge "Handle nested xtrace states (Quantum edition)"
Merge "Quantum/plugins/ryu: minor update for Quantum Ryu plugin"
Merge "Create tools/install_prereqs.sh"