* Refactor common network shared and platform specific code into package (part-1)
As per proposal #76 refactor common network shared and platform specific
code into sub-package.
https://github.com/ansible/proposals/issues/76
* ansible.module_utils.network.common - command shared functions
* ansible.module_utils.network.{{ platform }} - where platform is platform specific shared functions
* Fix review comments
* Fix review comments
* Handle common agrument in aggregate parameter for vyos module
* Add supoort to set parameter in aggregate to it's respctive
top level argument if value not provided in aggregate.
* Aggregate argument spec validation
* Documentation for aggregate
* Fix unit test failure