Set default pool value to system in gpfs driver

[OpenStack Infra]

https://review.openstack.org/119053
commit ce7c8dfba800874fec7411393e6799d81064f939
Author: Manojkiran <manojkiran@manojkiran.(none)>
Date: Thu Sep 4 18:03:57 2014 +0530

    Set default pool value to system in gpfs driver

    The configuration option gpfs_storage_pool is assigned with
    default value of None. And it gets initialized with None
    instead of system, where as the intention is to set the default
    value as system.

    This change sets default value of gpfs_storage_pool to system
    instead of None.

    DocImpact

    Needs an update in gpfs driver documentation, as this patch
    proposes a change in default value of configuration option
    (gpfs_storage_pool)

    Change-Id: Ib4c2a2e4593a940ae563d329fd59d2d5fe3d4520
    Closes-Bug: #1365474