apollo client:codegen does not respect config tag, only explicit —tag argument

**Intended outcome:**
I'm trying to push use codegen for my queries using `client:codegen`, and expect the CLI to respect the Apollo config

**Actual outcome:**
The command does not respect the config, and only works if I explicitly pass `--tag=mytag` to the command, otherwise it defaults to `@current`

**How to reproduce the issue:**

1. Have a service on the Apollo platform
2. Push a schema to a specific tag, i.e. `production`, `beta` etc.
3. Push a different, flawed/older schema to `current`
4. Try to codegen on your client using the `client:codegen` command and having set a tag in your config file
5. See the codegen fail, citing errors in the queries against the schema that match the differences between your `current` and `production` schemas.
6. Add `--tag=yourtag` to the CLI command, and see the codegen work

**Versions**
Using `apollo` v `2.4.4`.

**Extra info**
My config:

```
module.exports = {
client: {
service: 'myservice@production',
name: 'react-app',
version: '1.0.1',
includes: ['src/**/*.ts', 'src/**/*.tsx', 'src/**/*.graphql'],
},
engine: {
apiKey: '...,
},
};
```
The commands:
`"apollo-push": "apollo client:codegen",` or
`"apollo-push": "apollo client:codegen --tag=\"production\"",`

Start at the client:codegen CLI entry point and trace how the configured service tag is read versus the explicit --tag argument. Reproduce with the provided production/current schema setup and config, then verify that running client:codegen without --tag uses the configured tag and avoids the reported validation failures.

Written by the indexing model from the issue text.