Remove TARANTOOL_CLUSTER_COOKIE variable from cartridge dockerfile

[nickkkccc]

Now, unsetting the TARANTOOL_CLUSTER_COOKIE environment variable occurs on the first startup of the cluster if this environment variable is not explicitly specified in the .withEnv method and it is not present in the source system. When restarting cluster instances (from code, if the TARANTOOL_CLUSTER_COOKIE unset is not respected), the environment variable remains set and empty, which contradicts the usage logic.
Solution:

1. Remove the environment variable and set it only when the client has explicitly set it or it is in the environment variables of source system.
2. Unset the environment variable when restarting instances in the cluster (manually from the code).