Paco-models

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- Nothing changed yet.

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Added

- Error message when cfn-init files with !Sub and !Join can't be parsed.

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Fixed

- Ubuntu awscli install had extra whitespace which could stop up UserData.

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Added

- Added external_resource field for ICodeCommit.

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Added

- The TextReference class was renamed PacoRefernce and can now be passed `schema_constraint` with the
name or Schema that it must be a reference to.

- Support for `users` and `groups` in cfn-init. Invariant to prevent user name duplicating group name.

Changed

- Temporarily disable chmod 400 check on .credentials to support filesystems that don't have permissions.

- CodeCommit contains CodeCommitRepositoryGroups and CodeCommitRepostory group objects instead of a two-level dict.
Fixes docs and simplifies loader.

Fixed

- `Lambad.add_environment_variable` passes parent.

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Changed

- Schema clean-up, removed IMapping for all schemas that do not actually use it.

- Removed unused managed_udpates field for IApplication.