(gruntfile) Convert host/port/cert/key into env vars

[Someguy123]

For some reason, the listen host/port are hardcoded, as are the SSL cert/key.

I've converted them into environment variables with defaults, so they can be overriden by the user if they want/need to.

For example, maybe they only want it to listen on 127.0.0.1 / ::1

Or... they have something else already listening on port 8282 (which is the reason we made this PR!), allowing them to just change the listen port to something else.

Environment vars:

• DAPP_PORT (default 8282)
• DAPP_HOST (default 0.0.0.0)
• DAPP_CERT (default localhost.crt)
• DAPP_KEY (default localhost.key)

[denisgranha]

Hi @Someguy123 the port is the initial port for the Ledger API Bridge, if it's an used port it will iterate until it finds one open, at least initially that was the reasoning.

I think makes sense to configure it per environment, so I'm approving your PR

Thanks!