Pulled out mvndst parameters

********* PEOPLE *************
Primary reviewer: @sc932 

Reviewers: @suntzu86 

********* DESCRIPTION **************
Branch Name: denizokt_376_fixflakes
Ticket(s)/Issue(s): Closes #376 

********* TESTING DONE *************
make test
make style-test
made sure demos and pretty endpoints still work
Some issues here:

1. get/set_mvndst_parameters do not set this field.

2. this field should be private

3. this field should be settable through the ctor. you should default it to None, and then do:

  ```
if mvndst_parameters is None:
  self._mvndst_parameters = DEFAULT_MVNDST_PARAMS
else:
  self._mvndst_parameters = mvndst_parameters
```

also, either mvndst_params or _parameters. be consistent (and change the name of the default to match too)