Refactor config so that databaseImpl ('datastore') is under 'database' section

Currently, plugins that use the database declare the database impl (currently between Influx and MySql) as part of their plugin section in the AIT config. E.g.

plugins:
    - plugin:
        name: ait.core.server.plugins.openmct.AITOpenMctPlugin
        inputs:
            - telem_stream
        service_port: 8082
        debug_enabled: False
        database_enabled: True
        datastore: ait.core.db.InfluxDBBackend

...similar for the Data Archive Plugin.

However, once a DB instance is created, the plugin relies on the general 'database' section for database and access settings:

database:
     host: localhost
     port: 8086
     dbname: ait
     un: <username>
     pw: <password>

Now that there are two plugins that rely on a database, it seems that the datastore itself should also be under the 'database' section (and if there are cases of intentionally needing two separate DB types, then that info should be included in the plugin section?)

So it would be:

database:
     host: localhost
     port: 8086
     dbname: ait
     un: <username>
     pw: <password>
     datastore/impl/etc: ait.core.db.InfluxDBBackend

No files or tests are named. Start by locating the AIT configuration parsing and the plugin references to datastore, including the Open MCT and Data Archive plugins, then trace how the shared database settings are consumed. Done means the datastore implementation is configured under database consistently and both database-using plugins still initialize correctly.