Move version management scripts to releasetool

[garrettjonesgoogle]

In particular, these two scripts:

• https://github.com/googleapis/google-cloud-java/blob/master/utilities/bump_versions.py
• https://github.com/googleapis/google-cloud-java/blob/master/utilities/replace_versions.py

Key functionality that should probably be retained:

• The two-phase process of bump & replace
• The replacement of versions only at locations that are tagged
• A master version file (currently https://github.com/googleapis/google-cloud-java/blob/master/versions.txt )
• The tracking of last-released version and current version (which can be either last-released or a snapshot version)