Fix instantiating Vault Secret Backend during configuration (#17935)
Fix instantiating Vault Secret Backend during configuration (#17935)

When Secrets Backend are instantiated during configuration, not
all Airlfow packages are yet imported, because they need Secret
Backends. We have a weird cyclical relation between models,
configuration and settins which forces us to be extra careful
around configuration, settings and backends.

In this case top-level import of Connections by the Vault Secret
Backend triggered cyclic import problem (importing airflow models
require configuration to be fully loaded and initialized) but then
it could not be initialized because models needed to be imported
first.

The fix is to move Connections to be locally imported.
Acehaidrey/incubator-airflow,danielvdende/incubator-airflow,apache/incubator-airflow,lyft/incubator-airflow,bolkedebruin/airflow,lyft/incubator-airflow,lyft/incubator-airflow,nathanielvarona/airflow,apache/airflow,danielvdende/incubator-airflow,Acehaidrey/incubator-airflow,mistercrunch/airflow,apache/airflow,cfei18/incubator-airflow,bolkedebruin/airflow,cfei18/incubator-airflow,danielvdende/incubator-airflow,bolkedebruin/airflow,danielvdende/incubator-airflow,cfei18/incubator-airflow,mistercrunch/airflow,Acehaidrey/incubator-airflow,Acehaidrey/incubator-airflow,Acehaidrey/incubator-airflow,apache/airflow,apache/incubator-airflow,nathanielvarona/airflow,Acehaidrey/incubator-airflow,bolkedebruin/airflow,apache/airflow,apache/incubator-airflow,cfei18/incubator-airflow,mistercrunch/airflow,cfei18/incubator-airflow,nathanielvarona/airflow,nathanielvarona/airflow,danielvdende/incubator-airflow,lyft/incubator-airflow,nathanielvarona/airflow,nathanielvarona/airflow,apache/airflow,danielvdende/incubator-airflow,apache/airflow,mistercrunch/airflow,bolkedebruin/airflow,apache/incubator-airflow,cfei18/incubator-airflow