Minor improvements on the cookiecutter template

[nikParasyr]

Close #5

This PR makes some minor improvements:

• User can now define a floating ip pool to use when this is required ( it is quite often in openstack cluster to not allow to directly hook-up to external networks but only with fips )
• Vm destruction is handled by using the instance_id. This allows multiple instances with the same name (2 developers testing at the same time, or cicd etc)
• User can define a different network per instance (defaults to molecule which is defined in create.yml).
• User can define a different ssh_user per instance (defaults to cloud-user which is defined in create.yml). E.g a user has 5 instances, 4 using ubuntu and 1 centos. User can define an ssh_user per instance or change the ssh_user of create.yml to ubuntu and only define the centos user for the 1 instance that is left.

[nikParasyr]

@ssbarnea This does not require the molecule 3.0.4 to overwrite molecule.yml.

Let me know what you think