Add retries on networking errors and 5XX responses

[ljfranklin]

• om will now retry http requests twice in the event of networking errors or
  500 response codes

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[ljfranklin]

We've been intermittently seeing the following error in our CI:

processing product
beginning product upload to Ops Manager
 10.64 GiB / 12.60 GiB [==================================>------]  84.44% 1m34s
could not execute "upload-product": failed to upload product: could not make api request to available_products endpoint: Post https://pcf-optional.abalone-scale.gcp.releng.cf-app.com/api/v0/available_products: EOF

I'm hoping adding retries is a potential fix.