samples: PITR samples backup fix (#247)
samples: PITR samples backup fix (#247)

* samples: creates a backup using the current time

Instead of using the earliest version time of the database, uses the
current time (from spanner). If we used the earliest version time of the
database instead we would be creating an empty backup, since the
database we are backing up is not 1 hour old (default version retention
period).

* samples: addresses pr comments

Refactors create backup sample
googleapis/python-spanner,googleapis/python-spanner