Add NO_TEAR_DOWN opiton to regtest

Add NO_TEAR_DOWN option to keep environment for debugging or
demos. Also change the way to use virsh list as --name option
is not supported in ubuntu.
BTW, fixed a squid bug to skip caching xml files to avoid squid users
keep getting stale repo data.

Change-Id: I8c4000c8088ada15c1a5105105bded81ee8a1f70
Merge "Add NO_TEAR_DOWN opiton to regtest"