Set MPLCONFIGDIR env var for Matplotlib

matplotlib attempts to write configuration data to MPLCONFIGDIR, which is
currently unset. This change adds it as a class parameter and instructs Puppet
to ensure the directory exist. Also chmods both graphs and notebook dir to
0775.

Change-Id: Ib34147cf8befd8bfcd81a57c2b211942d37cd286
Merge "Set MPLCONFIGDIR env var for Matplotlib" into production