refactor channel mangling in config parser

the function `_add_new_channel` was too opaque and created things under the hood
the function `_get_new_node` was mutable
now it should be clear what's going on just from looking into the `parse()` flow
fix nodes with only networked channels

the nodes that had only network channels attached would not show in final config
it lead to various strange errors, like `zvm://` urls not being resolvable
Merge pull request #79 from pkit/bug_fix

fix nodes with only networked channels