[#724] Instance Configurator enhancements

* Create only users (not groups)
* Attach the security policies to the users
* Make an attempt to _GET_ the user first, and create it only if doesn't
  exist
* If the user already exist, request access key/secret for the user as
  we need it for writting the configuration file

NOTE: The instance configurator can be executed with existing object
up to 2 times. On the 3rd attempt it will fail due to Amazon IAM
limitis for (2 access keys per user)

We could try to delete the previous access key on each run, but we need
to consider the implications