[minor] add --welcome-message param to gitops-suite command

[tomklapiscak]

Description

Adds optional param:

      --welcome-message WELCOME_MESSAGE Default welcome message (displayed on login screen) to be configured (optional). NOTE: only use with MAS 9.1.7 or later.

This controls new feature in gitops to configure a default welcome message for display on the MAS login screen. For details, see ibm-mas/gitops#368

Testing

Confirm --welcome-message is optional

$ mas gitops-suite ... (no --welcome-message param specified)

generates ibm-mas-suite.yaml successfully with no welcome_message field

Basic test

$ mas gitops-suite ... --welcome-message "hello world"

generates ibm-mas-suite.yaml:

ibm_mas_suite:
  ...
  welcome_message: |
    hello world

Multiline string test

$ mas gitops-suite ... --welcome-message 'this
is
a
multiline
string'

generates ibm-mas-suite.yaml:

ibm_mas_suite:
  ...
  welcome_message: |
    this
    is
    a
    multiline
    string