use smb configured credentials from vagrantfile if available

The current SMB implementation will prompt the user for credentials if any synced folder entry lacks a username or password setting. Further, if a used does not explicitly create a synced folder with a `/vagrant` guest path, one will automatically be created (without credentials). This introduces at least two problem scenarios:

1. A user adds a synced folder for the current path but not named `/vagrant` and provides smb credentials:
```
config.vm.synced_folder ".", "/chocolateypackages", smb_username: "Matt", smb_password: "hot_mama"
```
2. A user adds synced folders for other paths all with credentials:
```
config.vm.synced_folder "./dog", "/a_folder_for_dogs", smb_username: "Matt", smb_password: "hot_mama"
config.vm.synced_folder "./ham_sandwich", "/a_folder_for_ham_sandwiches", smb_username: "Matt", smb_password: "hot_mama"
```

In both scenarios, the user will be prompted for smb credentials. This PR would:
* eliminate the duplicate `/vagrant` folder in the first scenario.
* reuse the Matt credentials for the `/vagrant` folder in the second scenario.