Create a new common collection named SimpleCollection

SimpleCollection is a collection of information that does not have a
Resource definition.
It can be useful in other situations so move it to a common place.

Signed-off-by: Aline Manera <alinefm@br.ibm.com>
Add API to list system users and groups

Signed-off-by: Aline Manera <alinefm@br.ibm.com>
Update users/groups verification based on new API

Use the APIs /host/users and /host/groups to check user input
That way we can check the input data at once instead of checking item by item.

Signed-off-by: Aline Manera <alinefm@br.ibm.com>
Remove unused code

With the new APIs /host/users and /host/groups we don't need the
User.exists() and Group() class so remove them.

Signed-off-by: Aline Manera <alinefm@br.ibm.com>
Update test cases

Properly test new APIs /host/users and /host/groups

Signed-off-by: Aline Manera <alinefm@br.ibm.com>