Provide a way (flag, samconfig.toml, etc.) to disable new version checking

### Describe your idea/feature/enhancement
I'd like to be able to disable checking for new versions. I'm running sam in a network isolated environment and every now and then builds will have a big traceback in them because sam couldn't reach pypi.

Provide a clear description. Ex. I wish SAM CLI would [...]

### Proposal

Add a global flag to disable version checking. Something like `--disable-version-check`.

Things to consider:
1. Will this require any updates to the [SAM Spec](https://github.com/awslabs/serverless-application-model)
I don't think so.