[2.5][FrameworkBundle] Redirect server output to /dev/null in case no verbosity is needed

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no 
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT

This should reduce I/Os
I don't see how it can change. There is no hook in the ``server:run`` command to allow changing the verbosity.