Don't try to `listdir` on a non existing `jid_root`.

Additionally, removed unnecessary brackets on logging messages and some PEP-8 formatting.
Use a default for multiprocessing if not set
Fix a misplaced `continue` on the grains matching logic which caused a traceback.

```
[21:18:46,004 [salt.utils.minions                          :200 ][ERROR   ] Failed matching available minions with grain pattern: planets:saturn
Traceback (most recent call last):
  File "/home/ubuntu/salt/salt/utils/minions.py", line 197, in check_minions
      }[expr_form](expr)
        File "/home/ubuntu/salt/salt/utils/minions.py", line 119, in _check_grain_minions
    str(match.lower()),
    AttributeError: 'list' object has no attribute 'lower'
```
Remove left over code.
Merge branch 'develop' of github.com:saltstack/salt into hotfix/fix-travis-again
Merge pull request #4068 from s0undt3ch/hotfix/fix-travis-again

Don't try to `listdir` on a non existing `jid_root`.
Merge pull request #4076 from torhve/develop

Use a default for multiprocessing if not set. This fixes #4075