Do not set cookie for prod node

[michalwski]

This PR doesn't set erlang cookie for the prod node build with rebar3's prod profile. Only dev nodes have erlang cookie set in their vm.args file and the test node.

This has the following implications:

• packages and docker image doesn't contain any erlang cookie. The cookie (if not set by the operator in vm.args) is randomized at startup (unless there is $HOME/.erlang.cookie file).
• when clustering MongooseIM, the operator needs to take care of erlang cookie and set the same cookie for every node in the cluster.

TODO:

• Update documentation with clear instruction where and why the cookie needs to be set when clustering MongooseIM (for instance in Cluster-configuration-and-node-management.md )
• Create a dedicated page in our docs describing how to secure the node(s). Firewall settings, which port should not be publicly available and why.

[mongoose-im]

6176.1 / Erlang 19.3 / small_tests / 05173b8
Reports root / small

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6176.5 / Erlang 19.3 / ldap_mnesia / 05173b8
Reports root/ big
OK: 1161 / Failed: 0 / User-skipped: 99 / Auto-skipped: 0

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6176.2 / Erlang 19.3 / internal_mnesia / 05173b8
Reports root/ big
OK: 1197 / Failed: 0 / User-skipped: 63 / Auto-skipped: 0

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6176.3 / Erlang 19.3 / mysql_redis / 05173b8
Reports root/ big
OK: 3067 / Failed: 0 / User-skipped: 231 / Auto-skipped: 0

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6176.6 / Erlang 19.3 / elasticsearch_and_cassandra_mnesia / 05173b8
Reports root/ big
OK: 469 / Failed: 0 / User-skipped: 8 / Auto-skipped: 0

---

6176.4 / Erlang 19.3 / odbc_mssql_mnesia / 05173b8
Reports root/ big
OK: 3070 / Failed: 0 / User-skipped: 228 / Auto-skipped: 0

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6176.8 / Erlang 20.0 / pgsql_mnesia / 05173b8
Reports root/ big / small
OK: 3102 / Failed: 0 / User-skipped: 196 / Auto-skipped: 0

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6176.9 / Erlang 21.0 / riak_mnesia / 05173b8
Reports root/ big / small
OK: 1432 / Failed: 0 / User-skipped: 61 / Auto-skipped: 0