Update write-pip-manifests pip freeze to use -l

Update write-pip-manifests pip freeze to use -l so that output
manifests do not include non-virtualenv version info. So that
subsequent rebuilds with DIB_PIP_MANIFEST_<service> will work.

Change-Id: I9c3b0ba1595db6e4d970dcf3ceb9a03211f864b0
Closes-Bug: #1305859
Merge "Update write-pip-manifests pip freeze to use -l"