13200: Fix python versioning for arvados-python-client and arvados-cwl-runner

13200: Fix python versioning for arvados-python-client and arvados-cwl-runner

New behavior:

1) If ARVADOS_BUILDING_VERSION is set, write that to _version.py

2) Else, try to import arvados_version, get version based on git
  tags+timestamp and write that to _version.py.  arvados_version.py is
  present in the source tree but not included in the package, so if we
  get ImportError, do nothing.

If neither (1) or (2) succeeds, take that to mean we're installing a
source package.  Leave _version.py alone.

Read the version back from _version.py and use that.

Arvados-DCO-1.1-Signed-off-by: Peter Amstutz <pamstutz@veritasgenetics.com>