lb4 discover unable to recognize MariaDB

@achrinza ci sta già lavorando.

Dal 24/3/2021.

## Steps to reproduce

1. Create a new project using `lb4 todolist-mariadb`
2. Create a new datasource using `lb4 datasource`

**mariadb.datasource.ts**
```
import {inject, lifeCycleObserver, LifeCycleObserver} from '@loopback/core';
import {juggler} from '@loopback/repository';

const config = {
name: 'mariadb',
connector: 'mysql',
url: 'mysql://node:123@localhost/todolist',
host: 'localhost',
port: 3306,
user: 'node',
password: '123',
database: 'todolist',
};

@lifeCycleObserver('datasource')
export class MariadbDataSource
extends juggler.DataSource
implements LifeCycleObserver {
static dataSourceName = 'mariadb';
static readonly defaultConfig = config;

constructor(
@inject('datasources.config.mariadb', {optional: true})
dsConfig: object = config,
) {
super(dsConfig);
}
}
```
3. Run `lb4 discover` but doesn't show any models!
```
$ lb4 discover
? Select the connector to discover mariadb
? Select the models which to discover (Press to select, to toggle all, to invert selection)
$
```

@loopback/* dependencies:
- @loopback/authentication: ^7.0.4
- @loopback/boot: ^3.1.1
- @loopback/build: ^6.2.7
- @loopback/context: ^3.13.1
- @loopback/core: ^2.13.0
- @loopback/metadata: ^3.0.4
- @loopback/openapi-spec-builder: ^3.0.2
- @loopback/openapi-v3: ^5.1.1
- @loopback/repository-json-schema: ^3.1.1
- @loopback/repository: ^3.2.1
- @loopback/rest: ^9.1.0
- @loopback/testlab: ^3.2.9
- @loopback/docs: ^3.19.0
- @loopback/example-hello-world: ^2.1.14
- @loopback/example-log-extension: ^2.1.14
- @loopback/example-rpc-server: ^2.1.14
- @loopback/example-todo: ^3.9.1
- @loopback/example-soap-calculator: ^2.4.7
- @loopback/service-proxy: ^3.0.4
- @loopback/http-caching-proxy: ^2.1.18
- @loopback/http-server: ^2.3.2
- @loopback/example-todo-list: ^3.6.2
- @loopback/dist-util: ^0.4.0
- @loopback/rest-explorer: ^3.0.4
- @loopback/eslint-config: ^10.0.3
- @loopback/example-express-composition: ^2.5.2
- @loopback/example-greeter-extension: ^2.1.14
- @loopback/booter-lb3app: ^2.2.14
- @loopback/example-lb3-application: ^2.5.2
- @loopback/example-greeting-app: ^2.2.12
- @loopback/example-context: ^2.1.14
- @loopback/repository-tests: ^0.14.2
- @loopback/health: ^0.6.1
- @loopback/authorization: ^0.7.4
- @loopback/rest-crud: ^0.8.18
- @loopback/security: ^0.3.4
- @loopback/authentication-passport: ^3.0.4
- @loopback/example-metrics-prometheus: ^0.4.4
- @loopback/metrics: ^0.5.1
- @loopback/model-api-builder: ^2.1.18
- @loopback/logging: ^0.4.4
- @loopback/example-access-control-migration: ^2.3.2
- @loopback/example-file-transfer: ^1.5.7
- @loopback/example-rest-crud: ^1.7.2
- @loopback/apiconnect: ^0.5.4
- @loopback/example-validation-app: ^1.8.2
- @loopback/cron: ^0.3.4
- @loopback/example-multi-tenancy: ^0.7.2
- @loopback/example-passport-login: ^1.8.1
- @loopback/authentication-jwt: ^0.7.4
- @loopback/context-explorer: ^0.2.4
- @loopback/express: ^3.1.0
- @loopback/example-todo-jwt: ^1.4.2
- @loopback/mock-oauth2-provider: ^0.1.12
- @loopback/pooling: ^0.3.4
- @loopback/typeorm: ^0.2.4
- @loopback/example-graphql: ^0.1.4
- @loopback/graphql: ^0.2.2
- @loopback/filter: ^1.2.1
- @loopback/rest-msgpack: ^0.2.3
- @loopback/example-binding-resolution: ^0.1.2
- @loopback/example-webpack: ^0.2.1
- @loopback/example-socketio: ^0.1.1
- @loopback/socketio: ^0.1.1
- @loopback/monorepo: ^0.1.1
```