GUI-1377 optimize dash status to exclude items who's tiles have been removed

optimize dash status to exclude items who's tiles have been removed

https://eucalyptus.atlassian.net/browse/GUI-1377
@dkavanagh: Is there a compelling reason why the TILE_MASTER_LIST isn't a simple Python list?  Couldn't we move it below the TILE_DISPLAY_NAMES dict and build up the master list via the following...

`TILE_MASTER_LIST = [key for key, val in TILE_DISPLAY_NAMES.items()]`

or if we want to keep it as a comma-separated string, it would be...

`TILE_MASTER_LIST = ','.join([key for key, val in TILE_DISPLAY_NAMES.items()])`

...to keep things DRY.