Allow for plugins to enhance how credentials are stored

[JTonda]

Allow plugins to influence how credentials are stored.
Credentials are stored in plain text by default.
Only one plugin that modifies this behavior can be installed at a time.
If credentials are stored securely and no secure credential management plugin is installed, inform the user.

Carried to sprint 2
3 points remaining

*Accept feature with PM

Tasks:

• Design new PMF architecture
  Assigned: @AHumanFromCA && @gejohnston && @lyzla01
  Status: Completed
• Load infrastructure plugins prior to the imperative overrides loader
  Assigned: @gejohnston
  Status: Completed
• Modify the Plugin Management Facility to allow for plugins that don't add groups
  Assigned: @gejohnston
  Status: Completed
• Defer logging of PMF messages until after the logger has been initialized
  Assigned: @lyzla01
  Status: Completed
• Load credential manager into config object passed into the overrides loader.
  The order, since we have no config, should be the last plugin wins. This will allow the default to win if there are no plugins. Implement a field changeable config file.
  Assigned: @AHumanFromCA
  Status: Completed
• Modify installer to write overrides to the settings.json file
  Assigned: @gejohnston
  Status: Discontinued
• Update system test
  Assigned: @lyzla01
  Status: Completed
• Create integration test scenarios for Imperative
  Assigned: @gejohnston
  Status: Completed
• Create integration tests in Imperative
  Assigned: @gejohnston
  Status: Completed
• Create infrastructure plugin example
  Assigned: @gejohnston
  Status: Completed
• Create infrastructure plugin doc
  Assigned: @gejohnston
  Status: In Progress
• Incorporate infrastructure plugin doc into official documentation
  Assigned:
  Status: Not Started
• Publish new version of Imperative
  Assigned:
  Status: Not Started
• Bump imperative version in Zowe CLI.
  Assigned: @gejohnston
  Status: In Progress
  Parent of Allow plugins to use the OverridesLoader imperative#21