Gluster: add the Shared File storage type.

The shared file and gluster disk templates should not report their disk
space information like file does, because they do not behave the same.

If a cluster pulls from the same, shared source of storage then it is
not correct, or useful, to report per-node disk availability, as the
information is not node-specific.

Additionally, this information should not be used by Ganeti to check
for system resource requirements. Say that our shared storage resource
has 20 GB out of 120 GB available and we want to migrate a 25 GB large
instance from node A to node B. Ganeti would calculate that migrating a
25 GB instance would change the cluster like this and prevent the move:

        DTotal  DFree      Calculated  Real
Node A    100G    20G  -->        45G   20G
Node B    100G    20G  -->        -5G   20G
Node C    100G    20G  -->        20G   20G

This change introduces the Shared File storage type for which no
per-node measuring of disk resources is made, following this rationale.
Gluster: implement getDefaultStorageKey

Adds Gluster specific code for getDefaultStorageKey