Can not delete share network that has no shares

[Valeriy Ponomaryov]

Currently tenants are unable to delete share_networks if there are existing share_servers. This doesn't make sense, because the share_server exists only for the share_network, and if the tenant is done with the share_network, then the share_server can go away safely.

Following changes should be done:
* If a share_network has no associated shares, then a delete request should succeed, regardless of whether a share_server(s) exist(s).
* Share_network deletion should trigger deletion of any associated share_server(s)

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/162138

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/162138
Committed: https://git.openstack.org/cgit/openstack/manila/commit/?id=6bdeb63c3b99a247ee5762f05b2c98334c1fc011
Submitter: Jenkins
Branch: master

commit 6bdeb63c3b99a247ee5762f05b2c98334c1fc011
Author: Valeriy Ponomaryov <email address hidden>
Date: Fri Mar 6 15:02:46 2015 +0200

    Always allow delete share-network when no shares exist

    Currently tenants are unable to delete share_networks if there are existing
    share_servers. This doesn't make sense, because the share_server exists only
    for the share_network, and if the tenant is done with the share_network,
    then the share_server can go away safely.

    Change-Id: I8925f3deba64da6b011516163e5554b8134c8c0a
    Closes-Bug: #1429123