Fix profile merge order to match Node.js SDK

[t1m0thyj]

Tested #144 with the sample config and profiles that we've been using. For the my_zosmf profile it is failing to load user and password:
Python SDK: {'host': 'example.com', 'rejectUnauthorized': True, 'port': 443}
Node.js SDK: {"port":443,"host":"example.com","rejectUnauthorized":true,"user":"admin","password":"password"}

The ProfileManager.load method loops through config files in the following order:

1. Project User Config (./zowe.config.user.json)
2. Project Config (./zowe.config.json)
3. Global User Config (~/zowe.config.user.json)
4. Global Config (~/zowe.config.json)

In the current implementation, profile properties are first loaded from each layer of the config individually, and merged together afterwards.

For the case of our sample config:

• The user and password properties are stored in the my_base profile of Project User Config (./zowe.config.user.json).
• This my_base profile is defined as the default base profile in Project Config (./zowe.config.json).
• At the time when we load Project User Config, we do not yet know that my_base is the default base profile, so the current logic will skip loading its properties. 😢

To fix this, I believe refactoring ProfileManager.load will be necessary. We should first merge all the config layers together, and then load profile properties as a single operation from the resulting merged object.

For a detailed explanation of how config layers should be merged together, see Zowe docs.

Originally posted by @t1m0thyj in #144 (comment)

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