Correct location of venv build logs

In [1] the intent was to have each venv build log
to its own log file. This makes troubleshooting a
lot easier due to the verbosity of the output in
the log - this at least separates each venv's
output. Unfortunately this extra CLI option was
supposed to be removed. This patch completes the
original intent.

[1] https://review.openstack.org/470757

Change-Id: I714d160a39beb870ba66db11a60cf0241ca770f3
(cherry picked from commit 12611a9)

defaults/main.yml

@@ -91,7 +91,6 @@ repo_build_venv_dir: "{{ repo_build_base_path }}/venvs/{{ repo_build_release_tag
 repo_build_venv_pip_install_options: >-
   --timeout 120
   --find-links {{ repo_build_release_path }}
-  --log /var/log/repo/repo_venv_builder.log
 repo_build_venv_command_options: >-
   {{ virtualenv_bin }}
   --always-copy