tests,acceptance: Respect LIBVIRT_DEFAULT_URI env

Only set `LIBVIRT_DEFAULT_URI` env if not already set.

Author: zeenix

travis/run-tests-acceptance

@@ -13,7 +13,10 @@ TEST_ARGS_DEF="-covermode=count -coverprofile=profile.cov -timeout=1200s"
 
 unset http_proxy
 export TERRAFORM_LIBVIRT_TEST_DOMAIN_TYPE=qemu
-export LIBVIRT_DEFAULT_URI="qemu:///system"
+if test -z "${LIBVIRT_DEFAULT_URI}"; then
+    LIBVIRT_DEFAULT_URI="qemu:///system"
+fi
+export LIBVIRT_DEFAULT_URI
 export TF_ACC=true
 
 set -x