Support nested attributes in import:node task

Dots are illegal characters as environmental variable names, therefore
we have to substitute something else in their place. Use a
double-underscore:

Instead of

    rake import:node NODE=node_key output.electricity=0.2

... run ...

    rake import:node NODE=node_key output__electricity=0.2
Cast all nested keys to Symbols in import:node task
Merge pull request #888 from quintel/import_dot_notation

Support nested attributes in import:node task