Clears adapters before registering to fix dbt-server cacheing behavior

[racheldaniel]

resolves #TIGER-28

Description

In implementing dbt deps logic in the new client/server, it became clear that running deps in the server results in cacheing of adapters-- such that the server needed to be stopped if additional packages needed to be added. This PR resets adapters in dbt.lib.get_dbt_config before registration.

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

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[jtcohen6]

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