Before this change, if a license in `emqx.conf` was of type
`key`/`file` and then changed to the opposite type, such change would
be saved to `{cluster,local}-overrides.conf`. When the node restarts
after that, the configs are merged naively and becomes invalid, as it
contains both the `key` and `file` keys, and crashes.
* Changed number of connections limit from 10 to 1000
* Added a 'deployment' field, so we know which is which when
a customer has multiple licenses
* Default expiration is 5 years
Zaiming (Stone) Shi