Blacklist some tmpfs's

Summary:
By default df shows stats on several ram filesystems. While
their stats are probably of interest, they still don't qualify as disks.
Since we're treating disk tracking somewhat specially, just blacklist
the specific devices we expect, namely ['df.dev', 'df.run',
'df.run-lock', 'df.run-shm', 'df.proc'].

Test Plan: pytest

Reviewers: carl, carlos, chase

Reviewed By: carl

CC: dashboard

Maniphest Tasks: T7885

Differential Revision: http://grizzly.memsql.com/D4205