1- What is the difference between those roles: "_member_" admin, ResellerAdmin and heat_stack_user
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A role is a personality that a user assumes to perform a specific set of operations. ( like an Admin or Reseller or just _member )
A role includes a set of rights and privileges. ( ???? and how to find out )
A user assumes that role inherits those rights and privileges.
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https://docs.openstack.org/admin-guide/cli-manage-projects-users-and-roles.html
OS documentation and look for "role"
https://docs.openstack.org/admin-guide/index.html
Authorization Model in OpenStack (keystone API V2.0)
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