Fix AttributeError in multinode jobs

I62006b7e636b1773f4f41709cfec1fd6daae3c4f introduced a bug where
the `skip_checks` method tries to access a compute client but the
compute client is not yet setup because the `setup_clients` has
not been called yet (`skip_checks` is always called before  `setup_clients`)
Hence the AttributeError.

The fix gets the number of compute nodes available through a newly introduced
config flag [1]. This way we don't dynamically discover the number of compute
nodes and we don't have to call any client.

[1]: I7d7230d100901179a1d1a0281aa8be990d00c02a

Closes-Bug: #1516561
Change-Id: I1b9474d2f0f0c2edea5ade4381c4c079804f10d5
1 file changed