Disable default usage of Keytar in Zowe CLI

[JTonda]

The effective requirement of keytar to use Zowe CLI is burdensome to adoption of Zowe CLI. By default Zowe CLI will not use or require keytar to securely store credentials in the operating system specific credential management. Future plugin infrastructure may be created such that keytar could be re-enabled via installing an infrastructure-only plugin.

Personas:
All
Use Case:
To foster adoption of Zowe CLI and reduce required prerequisites
Research Acceptance Criteria:

Acceptance Criteria:
A user can install and use Zowe CLI with plain-text profiles even if the keytar installation fails.
Profile migration guidance is provided
This change cannot alter Zowe CLI 1.0 behavior
Communicate to Che team whenever keytar removal is complete
Ensure Zowe CLI works in Che with non-keytar profiles
Ensure VSCode plugin will continue to function properly

Parent of #7
Parent of #9
Parent of #16
Parent of zowe/zowe-explorer-vscode#1

Child of https://rally1.rallydev.com/#/106710376756d/detail/portfolioitem/feature/249784471708

[JTonda]

From @AHumanFromCA

I already have a few ideas on how we can achieve this. I believe we could write our own post-install script for imperative that takes use of a flag to control the keytar install (much like how windows build tools can have flags provided to set the environment variables). We could then save this option in a config file that gets used by a base CLI to determine if keytar was installed or not. (This would determine whether keytar gets used by the credential manager or not). In the case of Zowe we could always set this to false so that keytar is never used.

This method would provide the most flexibility for both Zowe CLI and any other users of imperative.