[RbacBundle] Allow deep merging for roles_hierarchy

[aramalipoor]

Q	A
Bug fix?	no
New feature?	yes
BC breaks?	yes
Deprecations?	no
Fixed tickets	-
License	MIT
Doc PR	-

I needed to append some roles under administrator in my project. In fact I have a plugin system so that every bundle has its own config.yml, hence they might try to add addtional roles under adminstrator.

Since currently there is no way to dynamically modify Rbac information I thought I can use symfony's config merging as an easy way to extend rbac in my plugins, but to be able to do that I need to remove performNoDeepMerging from roles_hierarchy.

Do you guys think it's safe and right to do this on original RbacBundle?

/cc @pjedrzejewski @stloyd

[pjedrzejewski]

Sounds good, thank you Aram!