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

Change-Id: I9a62090ad2f900b9b150cacb9cb02b326cb46972
Merge branch 'prereqs2' into dt-test