Allow to define default zowe.yaml

[jackjia-ibm]

In addition to current example-zowe.yaml, we would like to define a new default-zowe.yaml with these facts:

• the default-zowe.yaml will be always loaded along with user customization.
• For most of the customization cases, the user doesn't need to work on full zowe.yaml, but only concentrate on small portion they really need to customize.
• We can easily add new configuration entries to zowe.yaml without the need of upgrading user customized zowe.yaml.