Modify entry script to execute command as is, if supplied to pipelines API directly.

[tneymanov]

Fixes the issue in #475

Tested via docker, gcloud and pipelines run.
Got the same error as customer with following command on older image, but passed with new image:
pipelines run action_input.json --scopes https://www.googleapis.com/auth/genomics,https://www.googleapis.com/auth/cloud-platform --zones us-west1-b

with action_input.json:
[
{
"imageUri": "gcr.io/tural-test-runner/gcp-variant-transforms:a53a8fa653d5110ac3da0d5ed71f1147dde3fd33",
"commands": ["/opt/gcp_variant_transforms/bin/vcf_to_bq", "--project", "tural-test-runner", "--input_pattern", "gs://gcp-variant-transforms-testfiles/small_tests/valid-4.0.vcf", "--output_table", "tural-test-runner:turalds.test1800", "--runner", "DataflowRunner", "--job_name", "vcf-to-bigquery", "--temp_location", "gs://tural-test-runner/temp", "--staging_location", "gs://tural-test-runner/staging"],
}
]

[coveralls]

Pull Request Test Coverage Report for Build 1687

• 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 87.816%

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Totals
Change from base Build 1683:	0.0%
Covered Lines:	7027
Relevant Lines:	8002

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💛 - Coveralls

[kemp-google]

I think you want to exec it instead so the return status is correct

[tneymanov]

Done.