Change `--tls-accept-all-certs` from String to switch in cb-env register

`cbenv-register` takes a flag `tls-accept-all-certs` that controls if all certs will work. Currently this is a String where it should be a switch. That is, if the flag is present then the value of the associated variable should be true, else it should be false. For an example see the `save` flag on the same command.

Start at the cbenv-register command's definition and compare the tls-accept-all-certs flag with the save flag on the same command. Done means the flag behaves as a switch: its associated value is true when present and false when absent.

Written by the indexing model from the issue text.