Add new variables and change defaults for some variables

[chrodriguez]

First of all, I have to say thank you for your work! It is the same module I was trying to write, and for this reason I am trying to commit some changes that can be useful for other users.

My changes propose making local_admin and ssh_private_key optiona values. And base volume resource creating be optional depending on other new variables.

When trying to use this module with my own configuration, I found myself with some limitations that think can be usefull to add and make this module more reusable:

• Storage management: added two variables to use existing base images an not always uploading a base image
  • base_pool_name: define an existing pool name
  • base_volume_name: define an existing base volume name
• Local user management: when this variable is empty do not create a local_user. Changed default value to make this a default behaviour.
• Test vm connection: in almost all cases users would use their default private key that may be loaded with ssh-agent. So default behaviour seems to be a null value, but allow to define a different one.

[MonolithProjects]

Thanks! lgtm