admin: Add hard-coded role support

Add RBAC to snf-admin. Currently, this works as following:

There are three roles (= astakos groups):

* admin-readonly: Users that have this role cannot perform any actions
  on items
* admin: Users that have this role can perform selected actions to items
* superadmin: Users that have this role can perform all actions to items

All users see the same item lists and item details. As far as the
actions that every role sees, they are hard-coded for now in the
respective action definitions, e.g. allowed_groups=['admin',
'superadmin'].

Fix apyrgio/synnefo#79
Merge branch 'feature-admin-roles' into feature-admin