Added model_specific defaults to params such as num_epochs, lr etc.
Applied Davide's changes.
Tested, resnet50 still gets built.
Here are before/after values of FLAGS.__dict__
```
{'datadir': '/tmp/imagenet', 'logdir': None, 'num_cores': None,
'batch_size': None, 'num_epochs': None, 'num_workers': 4, 'log_steps':
20, 'lr': None, 'momentum': None, 'target_accuracy': None, 'fake_data':
False, 'tidy': False, 'metrics_debug': False, 'model': 'resnet50'}
----------------------
{'datadir': '/tmp/imagenet', 'logdir': None, 'num_cores': None,
'batch_size': 128, 'num_epochs': 18, 'num_workers': 4, 'log_steps': 20,
'lr': 0.1, 'momentum': 0.9, 'target_accuracy': 0.0, 'fake_data': False,
'tidy': False, 'metrics_debug': False, 'model': 'resnet50'}
```
