Decouple admin-ui dependency from base keystone packages

[gautamsi]

This is related to #5649.

Another blocker for further extensibility is that base keystone packages depends on admin-ui which hinders the customisation of admin-ui by forking it.

I was trying to see if I could refactor some of the part myself and customise admin-ui, but realised that it is not possible due to dependency as well as use of admin-ui/system in cli.

I think we should also separate cli from the base keystone package

[bladey]

Thanks for providing this suggestion @gautamsi.

[gautamsi]

seems like we are doing opposite of this so may be we could archive this thread