Added support for KnpMenu v2-alpha

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | yes
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #1712
| License       | MIT
| Doc PR        | no

*WARNING* This PR makes the AdminBundle unstable as it depends on an unstable KnpMenu version. However, it should be pretty stable regarding features.

This comes for giving an answer to issue https://github.com/sonata-project/SonataAdminBundle/issues/1712 ; this will allow full support of MopaBootstrapBundle.