Make DoctrineBundle compatible with ORM 2.9

[beberlei]

In ORM 2.9 similar to DBAL 2.11 we introduce a new way of injecting the EntityManager into commands via registries/providers. These register the option em, which would lead to conflicts in the proxy commands due to redefinition of the option.

The same change was already made for dbal in 86d2469

Once ORM 2.9 is out we can think of increasing the doctrine/orm dependency from 2.6 to 2.9, since both support PHP 7.1. Then we can replace the whole proxy command code with proper dependency injection and shipping an entity manager provider.

This PR adds one test for one proxy command to test the variable entity manager option behavior. In addition since commands are services for a while already, the ContainerTest is also extended to fetch two out of the container

[ostrolucky]

Just a nitpick. This PR LGTM otherwise, thanks for doing it 👍 Merge with squash.

[ostrolucky]

return type should still be added for either Kernel or MockObject

[simPod]

Also note the parent option has default value while this has none.

->addOption('em', null, InputOption::VALUE_REQUIRED, 'Name of the entity manager to operate on', 'default')

I'm not entirely sure whats correct, though having default set as 'default' and not null disables AbstractManagerRegistry's functionality to use real default ($this->defaultManager) https://github.com/doctrine/persistence/blob/2.2.x/lib/Doctrine/Persistence/AbstractManagerRegistry.php#L143-L146 causing

Doctrine ORM Manager named "default" does not exist.

Issue for this doctrine/orm#8643

[beberlei]

@simPod hm thats annoying, i didn't want to make the manager name nullable in ORM if it could be avoided. :( Seems it is necessary just for backwards compatibility in Bundle

[simPod]

Understand. I'm using only one EM for now (used to have more so it has custom name). Though it's not working with this change and it requires to specify em option which is also very annoying when there's only one EM 😄