LTI-379: rake task to bulk add settings

[Mariam05]

A new rake task was added to allow settings to be added to a tenant in bulk.

The rake task format is: rake tenant:settings:bulk_add[tenant-uid, path-to-json-file]

For example, if these are the current settings for tenant 'test':
{ "ext_params" => { "join" => { "key1" => "value1"}}}

And this is the content of my json file:

{ 
"ext_params": { 
      "join": {  "key2":  "value2" },
      "create": { "key3": "value3" }
      },
"enable_shared_rooms": "true"
}

then the result would be:

{ 
"ext_params" => { 
      "join" => { "key1" => "value1", "key2" => "value2"},
      "create" => { "key3" => "value3" }
      },
"enable_shared_rooms" => "true"
}