allow pipeline class to specify "MAX_ATTEMPTS"
allow pipeline class to specify "MAX_ATTEMPTS"

Enables a pipeline class to provide its own value for
`_DEFAULT_MAX_ATTEMPTS`.

Loudr/appengine-pipelines,Loudr/appengine-pipelines,Loudr/appengine-pipelines,Loudr/appengine-pipelines,Loudr/appengine-pipelines