Rename resource "ShortTermRetentionPolicy"->"BackupShortTermRetentionPolicy" to align with "BackupLongTermRetentionPolicy"

[adeal]

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[jaredmoo]

It also aligns with backupShortTermRetentionPolicies in the URI :)

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Azure/azure-sdk-for-python#2285

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AutorestCI/azure-libraries-for-java#9

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[jhendrixMSFT]

There are a few examples missing response bodies for 202 status codes, see https://travis-ci.org/Azure/azure-rest-api-specs/jobs/382401500. I know it wasn't introduced in this PR, is there an issue tracking fixing them? It would be awesome to have the model validator clean.