Cleanup ec2/metadata/osapi address/port listen config option help

Commit d88cfbd changed the help for the osapi_compute_listen_port config
option but the grammar could be cleaned up a bit for not only that
option but the ec2 and metadata API IP address/port listening options in
nova.service, so this patch cleans them all up.

Change-Id: I66533d576046141bd3cd2f265dc2cfa6383c738d
Merge "Cleanup ec2/metadata/osapi address/port listen config option help"