Feature add grains support to master and roster

This adds the ability to specify custom grains on a salt-ssh target "minion".

In master conf:
```
ssh_grains:
  - somegrain
```

In roster:
```
testsrv:
  host: testsrv.local
  user: vagrant      
  sudo: True         
  grains:
    roles:
      - role1
      - role2
```

There is a preference order in play. The roster will overwrite master's conf, which in turn will overwrite grains on the target "minion".