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()