collect coverage data from ci nodes before teardown
collect coverage data from ci nodes before teardown

Fixes #2071.

weaveworks/weave,n054/weave,weaveworks/weave,paulbellamy/weave,paulbellamy/weave,weaveworks/weave,n054/weave,n054/weave,paulbellamy/weave,n054/weave,weaveworks/weave,n054/weave,paulbellamy/weave