CB-7447 Add --globalplugins option to createmobilespec.js

Add option to createmobilespec.js to use local platforms and tools, but use
released version of plugins. This is meant for running tests for a
platforms/tools release.
[CB-7470]createmobilespec.js --global / --globalplugins shouldn't clear $HOME/.cordova/commonlibs
[CB-7469]createmobilespec.js --global is broken since the new plugin test infrastructure was added

Updated script works on amazon-fireos as well with --global flag.