splits master.cfg according to INFRA's buildbot configuration

* split master.cfg into master.cfg, projects.conf and cordova.conf
* integrate changes from CB-8091 and CB-8099 commits
* update readme according to new configuration files structure
Multiple improvements

* Put all config-related routines into MedicConfig class()
* Move cli_steps and plugman_steps into PlatformTestBase since they are common for all builders
* introduce get_default_factory() method for PaltformTestBase to simplify creating build factories
* replace windows8 -> windows in platform names
Replace comment with module docstring
fixes createmobilespec failure due to lack of cordova-plugman repo

* Add builders using get_default_factory()
Improvements and unnecessary steps cleanup

* remove prepare cordova-lib and link cordova-lib steps from cli_steps group since `coho npm-link` do the same job
* remove build_cordovajs_steps and copy_cordovajs_steps since this routines is handled by mobilespec now
* remove unused writejson.js file
* remove whitelist rules removal logic from updateconfig.js since windows platform handles whitelists correctly
Fixes some of PEP8 issues
INFRA-8588 Split medic Buildbot configuration according to Apache's Buildbot config

Merge branch 'master_cfg_rework'