Allow having mta send mail directly.

Initial motivation was wanting to ensure that `relayhost = ` was set.  As a side effect this also adds a new value to the postfix::config ensure parameter called 'blank'.  `ensure => 'blank'` will use augeas clear which allows blanking any config option.

Example changes:
This would say send mail directly instead of through a relay host
```puppet
class { 'postfix':                                                           
  relayhost     => 'direct',                                       
  smtp_listen   => '0.0.0.0',                                                
  mydestination => '$myorigin, myapp.example.com',                           
  mta           => true,                                                     
} 
```

The underlying resource for that option is essentially:
```puppet
postfix::config { 'relayhost': ensure => 'blank' }
```

having a relay host that's blank or just a space doesn't work with augeaus. You have to use clear command in augeas in order to get the desired option.