[cloudbuild] support for steps[*].allowFailure

[bradrydzewski]

https://cloud.google.com/build/docs/build-config-file-schema#allowfailure

In a build step, if you set the value of the allowFailure field to true, and the build step fails, then the build succeeds as long as all other build steps in that build succeed.

steps:
- name: 'ubuntu'
 args: ['-c', 'exit 1']
 allowFailure: true
steps:
- name: 'ubuntu'
 args: ['echo', 'Hello World']

This is supported in the v0 yaml so we just need to add the mapping.

                  failureStrategies:
                    - onFailure:
                        errors:
                          - AllErrors
                        action:
                          type: Ignore

Update added support for allowFailure; need to add downgrade logic

[bradrydzewski]

mapped to v1 failure strategies. the downgrade mapping is not yet complete, though
https://github.com/drone/go-convert/blob/fa88d1e840237b74941669da12af6d5692a473c4/convert/cloudbuild/convert.go#L311:L323