Improve the node driver interfaces validation output

This patch improves the output of the node driver interfaces validation
to return the reason why the interface failed to validate.

The new structure looks like:

{'deploy':  {'result': True},
 'power':   {'result': False, 'reason': 'Invalid IPMI username'},
 'console': {'result': None, 'reason': 'not supported'},
 'rescue':  {'result': None, 'reason': 'not supported'}}

The patch also remove an unneeded log that was logging the errors for
each interface, every time the validation was called.

Change-Id: Id5d8aa1bd150b7d85f359471daccfacb6d9e2ede
Closes-Bug: #1263210
Merge "Improve the node driver interfaces validation output"