broker: use direct I/O to read metadata

In some cases a host's filesystem cache may hold stale data from parts
of the shared metadata file not updated by that host.  This can lead to
the host falsely concluding that other hosts are down.  Using direct
I/O for the read operations avoids this problem by fetching the latest
data directly from the NFS server.

Change-Id: I296977000ffa3fff3f9391f93a8b4f3f519eae4e
Bug-Url: https://bugzilla.redhat.com/1014241
Signed-off-by: Greg Padgett <gpadgett@redhat.com>