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

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

Change-Id: I9a62090ad2f900b9b150cacb9cb02b326cb46972
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
Merge branch 'prereqs2' into dt-test
Merge branch 'nested-xtrace2' into dt-test
Merge branch 'database-service' into dt-test