Add script to delete extra appengine services

[stephen-soltesz]

AppEngine has a hard limit of 210 service versions across a project. This change adds a small script that deletes all service versions after the first 9 that are non-serving. The number of older versions to preserve is arbitrary; more than 1 to allow rollbacks is the main intention. 9 is conservative number. This change was inspired by an alert in staging about there being too many service versions.

Limitations / caveats:

• The first time this script runs, it will delete many older versions and will take longer than steady state. At the limit, it should delete no more than one per service.
• This script applies to all app engine services but will only run on ETL deployments.

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This change is 

[coveralls]

Pull Request Test Coverage Report for Build 7056

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 64.217%

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Totals
Change from base Build 7050:	0.0%
Covered Lines:	3844
Relevant Lines:	5986

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💛 - Coveralls

[stephen-soltesz]

It works!