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**Apache Airflow version**: 2.0.0b2

**Environment**: Docker

- **Cloud provider or hardware configuration**:
- **OS** (e.g. from /etc/os-release):
- **Kernel** (e.g. `uname -a`):
- **Install tools**:
- **Others**:

**What happened**:

In current 2.0.0b2, `airflow connections add` is not handling invalid URIs properly. 

For example, running CLI `airflow connections add --conn-uri xyz conn_dummy` will
- create the connection “conn_dummy” successfully (which should not be, IMO)
- in the connection created, it only has one attribute filled, which is “schema”, but the value added is “yz” (the value we provide is "xyz", i.e., the 1st element is removed. because of https://github.com/apache/airflow/blob/master/airflow/models/connection.py#L213)

<img width="552" alt="airflow_0284e070228e___" src="https://user-images.githubusercontent.com/11539188/99155673-519dca00-26ba-11eb-9517-31c1a238476f.png">


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**What you expected to happen**:


In my opinion, a validation should be done first to ensure the `conn-uri` provided is valid (at least have “scheme” and “netloc” available in the `urlparse` result), and reject if it's an invalid URI.

<!-- What do you think went wrong? -->


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tests/cli/commands/test_connection_command.py
(
            'mssql_default',
            'mssql',
        ),
        (
            'mysql_default',
            'mysql',
        ),
        (
            'pinot_broker_default',
            'pinot',
        ),
        (
            'postgres_default',
            'postgres',
        ),
        (
            'presto_default',
            'presto',
        ),
        (
            'sqlite_default',
            'sqlite',
        ),
        (
            'vertica_default',
            'vertica',
        ),
    ]
    def setUp(self):