CB-6909 shellCmd and shellRunParam in master.cfg use values for master, fails on Windows slaves if OS X master

shellCmd and shellRunCmd defined in master.cfg depends on the OS of the master, so if you have a OS X master and Windows client (or vice versa) a lot of steps will fail.
For some steps, like in wp8 and windows8 sections, we know they'll always run on Windows so we could always use 'cmd'. But in common sections or for a platform like android, it needs to work on both.

Fix for [CB-6909](https://issues.apache.org/jira/browse/CB-6909)