Configure Gitaly socket_path, listen_addr, prometheus_listen_addr via config.toml

The configuration format of Gitaly is shifting from environment variables to a config.toml file.

We currently have some automagic code in omnibus that changes lower case gitaly['foobar'] options into GITALY_FOOBAR environment variables. This code needs to be removed. In its place we need three 'new' options:

gitaly['socket_path'] = '/var/opt/gitlab/gitaly.socket' # used to be GITALY_SOCKET_PATH
gitaly['listen_addr'] = nil # should not be rendered by default
gitaly['prometheus_listen_addr'] = 'localhost:9236' # not sure what default should be. used to be GITALY_PROMETHEUS_LISTEN_ADDR

Note that there is some auto-configuration code that touches GITALY_SOCKET_PATH. That code needs to be updated to work with node['gitlab']['gitaly']['socket_path'].

There is a parallel config.toml issue here: https://gitlab.com/gitlab-org/omnibus-gitlab/issues/2182