role delete: check for r/o role and role in a group in model

The following (existing) statement in MiqUserRole class:

    has_many :miq_groups, :dependent => :restrict

will prevent from deleting a role in a group, the 'before_destroy'
statement is to prevent a read only role from being deleted.

Therefore, there's no need to be checking for read only role
and a role in a group in the controller code.

https://bugzilla.redhat.com/show_bug.cgi?id=1126536
group delete: check for a r/o group and group used by users in a model

This fix is:
* to make sure during group removal, the check for a read-only group,
as well as a group still in use by user(s) is being made in the model,
rather than the controller.
* to simplify the (controller) code

https://bugzilla.redhat.com/show_bug.cgi?id=1126536
Merge pull request #390 from mzazrivec/role_group_check_in_model

Check for used & read-only roles & groups in model rather than controller