Raise error if a group and host is created with the same name

[AlanCoding]

ISSUE TYPE

• Feature Idea

SUMMARY

Create an inventory, inside that inventory create a host with name "foo". Then create a group with name "foo".

This breaks playbooks.

We should prevent the creation of the group with name "foo" because a host by that name exists, and return a 400 error message with details in this case.

Related #3384

[kdelee]

verified, tried it a few ways:

• made group first, tried to add host to inventory or in the group itself, both denied saying a group already has the name
• made the host first, tried to add the group and got an error saying a host already existed.

closing as verified, thanks!