This PR fix multiple problems -
1. This PR fix the env manifest fields that were organized in incorrect manner.
2. Also resolves aws#4032
3. `copilot env init` with private subnets create env manifest with following field
```
http:
  private:
```
This should only be created when there is at least one parameter inside private is enabled. This PR fix this nit as well.

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Fixes second issue in aws#3553.

Here strings (`<<<`) don't work with `sh`. This change makes the pipeline buildspec a bit more universal.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the Apache 2.0 License.