Integration Tests - Use default credential chain not requiring SA

[gbhat618]

Currently the integration tests expect to create a service account and export it to environment variables

GOOGLE_CREDENTIALS_FILE or GOOGLE_CREDENTIALS
GOOGLE_SA_NAME

it is bad practice and should try to use the default credentials instead.

Currently simply using the default credential path ~/.config/gcloud/application_default_credentials.json is not working - need to make appropriate changes to the credential reading and test automations.

See https://github.com/jenkinsci/google-compute-engine-plugin/pull/495/files/79b8ffdb9a7ad456b7ea013e88802f2ea3cd1046#r1895805984

[gbhat618]

when credentialsId==null get the credential by getApplicationDefault

[gbhat618]

Also as noted in here #495 (comment)
create new issue as per the status of client library and possibility to change to a better maintained client library