Rather than using tag -f, just test if a tag exists before creating it.

I was using tag -f to allow us to (re)create a tag even if it already
exists.  But that was still giving us problems at 'git push --tags'
time; since I redid the tag I guess I would have needed to specify -f.

But the tag contents don't actually change (I recreate the tag with
the same contents as before) so can avoid all that trouble, at the
cost of one extra line of code.  Now I check if the tag already exists
before trying to create it.

Auditors: chris

Test Plan:
Ran
   jenkins-tools% python
   >>> import deploy_pipeline as d
   >>> d._tag_release({"VERSION_NAME": "test", "GIT_SHA1": "HEAD"})
   >>> d._tag_release({"VERSION_NAME": "test", "GIT_SHA1": "HEAD"})

and saw that the second call ran without error.  I then ran
   webapp% git tag -l          # Note: in the webapp directory!
and saw gae-test at the bottom.  (I then ran 'git tag -d gae-test'...)