Introduce node and group level vars

Variables can now be fed to model from multiple locations. In order of
preference:

1) node (from source)
2) group
3) global

In a model vars should be accessed via 'vars' helper method though it is
not required. Helper method ignores nil values so care needs to taken
when designing model behaviour.

Support for node level vars is currently available on sql source via
'vars_map' configuration.

Following example populates node vars 'enable' and 'somevariable' from sql
columns 'var_enable' and 'var_somevariable'

   sql:
     adapter: sqlite
     file: /home/aakso/.config/oxidized/sqrouter.db
     table: nodes
     map:
       name: hostname
       model: model
       group: node_group
       username: username
       password: password
     vars_map:
       enable: var_enablepw
       somevariable: var_somevariable

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