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.
nova_compute: remove ternary statement
Merge pull request #6230 from joshuaconner/bugfix_nova_computer_partial_server_name_match

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