Config file for consistently applying environment variables

See MB-10246 for similar thoughts.

It is, at times, necessary to modify Couchbase by setting environment variables before start or restarting the couchbase-server service. In some environments it can be challenging to appropriately configure the environment variables, since simply performing `export VAR=value` and restarting the service will not apply the variable. Why this is, I am not certain, but I believe it has to do with the daemon forking and 'couchbase' user account used.

Instead, it is necessary to instruct users requiring the editing of environment variables to edit the `/opt/couchbase/bin/couchbase-server` script and place the `export` statements in, for example, the `_start` function itself.

Customers have requested that we provide a configuration file for managing the environment variables. This is analogous to an /etc/default file for the couchbase service.

This ticket differs from MB-10246 in that that ticket has a wider scope, beyond environment variables and things that could be done simply by sourcing a shell file.