Fix seed_master key file name

As per https://github.com/saltstack/salty-vagrant/pull/98 and https://github.com/saltstack/salty-vagrant/issues/97 the expected file name for minion is 'minion', not 'seed-minion.pub'
Merge pull request #2359 from viq/patch-1

provisioners/salt: fix seed_master key file name