Change: Unify Envvar names configuring REVA gateway address

We've renamed all envvars configuring REVA gateway address to `REVA_GATEWAY`, 
additionally we renamed the cli parameters to `--reva-gateway-addr` and adjusted the description

https://github.com/owncloud/ocis/pull/2587
https://github.com/owncloud/ocis/issues/2091

