QA: Fix duplicated OOB tests

Patch f55312bd added the OOB tests to TestClusterVerify, which is not
actually a test for cluster verify, but a runner for cluster verify
that is called multiple times, for each instance type, etc. This led
to running the OOB commands multiple times, which is painful
especially as this is a slow test.

The patch moves this to a separate test, that is run only once.

Furthermore, the way that data files are copied around is very
inefficient: touch + mv + chmod + mv + rm for each node (5 times
number of nodes), whereas it could be simply: touch on master, chmod
on master, cluster copyfile, chmod on master, cluster copyfile,
cluster command rm, i.e. only 5 fixed ssh calls to the master. The
code is changed as such, for increased speed.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>