Make _extra_keys a property of ModelBase

Currently  _get_extra_keys() used as as function.
Changing it to read only property makes class interface more cleaner
and reliable
We don't have to explicitly call it and it can't be accidently overriden
by assigning of an instance variable

Change-Id: I236408be953f299854fce80aa38e24708c13a60a
Remove start index 0 in range()

Remove the useless arg ("start index" = 0) in files, since its default
value is 0, to make code cleaner.

Fixes bug #1259744

Change-Id: I02fc3d96cacdbae1be9e9283362bb87518d97a7c
Merge "Make _extra_keys a property of ModelBase"
Merge "Remove start index 0 in range()"