Added --file-root option for local salt run

The --file-root option allows salt to run an arbitrary SLS file anywhere
on the filesystem without having to have a Salt master, or states
installed in /srv/salt.  For example, to run a .sls file located at
`/home/user/foo.sls` the following command can be used:

```
salt-call --local --file-root=/home/user state.sls foo
```

Additionally, an entire state tree can be used with this option.  For
example, the sls file `/home/user/states/bootstrap/init.sls` can include
`service.foo` at `/home/user/states/service/foo.sls` with the command:

```
salt-call --local --file-root=/home/user/states state.sls bootstrap
```
Merge pull request #7861 from rca/salt_local_file_root_option

Added --file-root option for local salt run