'+' config prefix handling whitespace

[jmriego]

resolves #3494

Description

There is not really supported to add a whitespace after the plus sign in the configuration yml but the error is not intuitive. There was a fix for the eager rendering of macros and this extends this fix for when there's a space between the plus sign and the key (eg. + post-hook)

Checklist

• I have signed the CLA
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• I have updated the CHANGELOG.md and added information about my change to the "dbt next" section.

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Thank you for your pull request and welcome to our community. We could not parse the GitHub identity of the following contributors: Jose Valenzuela.
This is most likely caused by a git client misconfiguration; please make sure to:

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[cla-bot]

Thank you for your pull request and welcome to our community. We could not parse the GitHub identity of the following contributors: Jose Valenzuela.
This is most likely caused by a git client misconfiguration; please make sure to:

1. check if your git client is configured with an email to sign commits git config --list | grep email
2. If not, set it up using git config --global user.email email@example.com
3. Make sure that the git commit email is configured in your GitHub account settings, see https://github.com/settings/emails

[cla-bot]

Thank you for your pull request and welcome to our community. We could not parse the GitHub identity of the following contributors: Jose Valenzuela.
This is most likely caused by a git client misconfiguration; please make sure to:

1. check if your git client is configured with an email to sign commits git config --list | grep email
2. If not, set it up using git config --global user.email email@example.com
3. Make sure that the git commit email is configured in your GitHub account settings, see https://github.com/settings/emails

[jtcohen6]

Thanks for this one @jmriego!

It looks like the Redshift tests failed for an unrelated reason. I'll get that sorted, but in the meantime this is good to go.