Always bind-mount pypi mirror if dir exists

It may be desirable to specify file://tmp/pypi as one of the list
of mirrors specified by PYPI_MIRROR_URL, so the local pypi mirror
should be mounted as long as the directory exists.

Even though the path is mounted, it is only used by pip if there is
no specified PYPI_MIRROR_URL, or if one PYPI_MIRROR_URL entry
specifies file://tmp/pypi.

The pypi element previously assumed that a local mirror would be
a complete offline mirror. This change allows for a mixture of
local mirror plus remote mirrors. This means that the local
mirror could (for example) contain a few locally built pip packages
from arbitrary git checkouts for gate testing of those projects.
In this scenario, most packages would be downloaded from the
configured remote mirrors.

This change is required for synchronous gating on the os-*-config
tools.

Change-Id: I0f6d153c06993ffd0d90bd8ca1717462e8b6b541
Merge "Always bind-mount pypi mirror if dir exists"