nova_compute: fix for partial match b/w params['name'] and an existing name

When there is an Openstack instance that has a name that's a partial match for module.params['name'], but a server with name module.params['name'] doesn't yet exist\*, this module would fail with a list index out of bounds error. This fixes that by filtering by exact name and only **then** getting the server from the list if the list is still not empty.

\* For example, a server with name 'nginx-prod' exists, and module.params['name'] = 'nginx'.