Conditionally mark test_attach_detach_volume as slow

Based on average test times from the last 300 runs
of the tempest-full job [1] this change marks the
test_attach_detach_volume test as slow but only if
SSH validation is enabled, which it is in tempest-full
but might not be in other jobs that run it, for example
in refstack which uses this test for interoperability
certification.

[1] http://paste.openstack.org/show/745114/

Change-Id: I47710321d8dfb0345c793e66bd09a5dd1933557b
Related-Bug: #1783405
1 file changed