[FEATURE] multidomain-domainnames (ffnord#59)

* Implemented util.read_domainfile

This reads a given json file containing a dict with bat_device names as
keys and domain_names as values.

* Provided an example domainfile.

* Added a new parameter to spcify where to read the domainfile from.

It's optional.

* Altered the domain_code-provider in order to support domaincode-files.

Given a dcf-path, the provider tries to return the domaincode related
to a batadv_dev, like trier did earlier in their code.

If it cannot find a match, it returns whatever was provided as
'domaincode' via '-n' in the commandline, which makes it effectively a
default value for unkown domains and a nice fallback.

* updated util.read_dumainfile

removed unnecessary return variable

Co-Authored-By: Martin Weinelt <[email protected]>

* updated domainfile example to reflect domaincodes

* util: removed redundant line

* README: add domain_code-file documentation

* Fixed type in 'return', however it got there.

* Added per batman iface domaincode overrides

* readme: reflects per batinterface domaincode

* domaincodefile: changed format to one-key-dict

The file now holds a key called 'domaincodes' and below the assignments.
This allows a transition from domaincodefile to configfile in the
future, if wanted.

Co-authored-by: Martin Weinelt <[email protected]>