Allow MB to be specified manually without a pageant dir

[tavinathanson]

@tavinathanson:

i think this is where you'll need to change something for lung, somehow making this configurable to not load the ensembl_coverage loader https://github.com/hammerlab/cohorts/blob/4718cb08ccb9f0c08af159b30e11dcaac09b5cfb/cohorts/functions.py#L91 maybe e.g. if 'MB' column exists use that else join that'll call load_ensembl_whatever which fails if there's no pageant dir

@arahuja:

won’t it first check that the key exists? oh ok so the key does exist and the it’ll call load and that will fail with a path