Created a way to let users change the layout of the rooms.

To do this, a new model was created to receive the options from the room. In this model a user can set the default layout who he wanted. A new function was created in this model to return all the layouts allowed in the server, for now this function just return a list of fixed layouts.

Refs #962
Update rooms_controller.rb

Adding a restriction for allowed parameters on rooms. Now the parameter default_layouts can be modified.

Refs #962
Initial changes to use dynamic config.xml to join meetings with a specific configuration

Created a new function in bigbluebutton_room model to generate a new token with all configurations changed by the user.

Refs #962
Pass configToken to join calls, fully functional

Made the last changes to pass the configToken to join calls. Also added the
complete list of layouts available in BigBlueButton/Mconf-Live.

refs #962