Correct policy action for backup export volume endpoint

This PS corrects the policy action for exporting a volume
backup from "backup:backup-export" to "backup:export-import".
The former is causing the related RBAC test to fail [0] and
the in-code policy documentation confirms that the latter is
correct [1].

[0] http://logs.openstack.org/08/518208/3/check/legacy-tempest-dsvm-patrole-multinode-admin/5f129df/job-output.txt.gz#_2017-11-08_01_02_55_782843
[1] https://github.com/openstack/cinder/blob/fb27334719fb612d2d5386b7d9de374d4a415d81/cinder/policies/backups.py#L127

Change-Id: I10dd9cb8134a2b83cf92131c1b28a58881757f32
1 file changed