Update git submodules
* Update bifrost from branch 'master'
- Merge "Set min version of tox to 3.2.1"
- Set min version of tox to 3.2.1
As recommended, since version 3.2.0 tox switches pip invocations
to use the module -m pip instead of direct invocation.
We set min version to 3.2.1 [1] to also fix the behavior of
--parallel--safe-build
[1] https://tox.readthedocs.io/en/latest/changelog.html#v3-2-1-2018-08-10
Change-Id: I8ff1d620d5ab7608847fd4ac03a06168e3d1db2d
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