Split PV, one for Postgres and for Galaxy Data

[sneumann]

Hi, before tackling #22 I'd like to work on splitting the PV into
one for Postgres /var/lib/ and one for /export Galaxy data.
Then we can also try to apply fixes found in these comments, updated in 2018:

https://bugzilla.redhat.com/show_bug.cgi?id=1512691
https://bugzilla.redhat.com/show_bug.cgi?id=1518710

Additional info: When the following volume options are turned off the benchmark runs smoothly without errors and delivers the expected output above.
  performance.open-behind: off
  performance.write-behind: off

[andersla]

@sneumann Should I add only those two performance options on the gluster-disk.
I think It is a good idéa to put those as default on all gluster PVs? Or?