Allow user to run install_flux.sh with a prompted password

Package Merge Request

Package Changes

This MR allows the user to enter their password without having to expose it on the command line. It also prompts for the username if one is not entered.

For Issue

#1576 (closed)

Closes #1576 (closed)

Test Case

➜  bigbang git:(1576-install-flux-passwd-prompt) ✗ ./scripts/install_flux.sh
Please enter your Registry1 username: Ben_Francis
Please enter your Registry1 password: REGISTRY_URL: registry1.dso.mil
REGISTRY_USERNAME: Ben_Francis
Creating flux-system namespace so that the docker-registry secret can be added first.
namespace/flux-system unchanged
Creating secret private-registry in namespace flux-system
secret/private-registry created
Installing flux from kustomization
namespace/flux-system configured
customresourcedefinition.apiextensions.k8s.io/alerts.notification.toolkit.fluxcd.io created
customresourcedefinition.apiextensions.k8s.io/buckets.source.toolkit.fluxcd.io created
customresourcedefinition.apiextensions.k8s.io/gitrepositories.source.toolkit.fluxcd.io created
customresourcedefinition.apiextensions.k8s.io/helmcharts.source.toolkit.fluxcd.io created
customresourcedefinition.apiextensions.k8s.io/helmreleases.helm.toolkit.fluxcd.io created
customresourcedefinition.apiextensions.k8s.io/helmrepositories.source.toolkit.fluxcd.io created
customresourcedefinition.apiextensions.k8s.io/kustomizations.kustomize.toolkit.fluxcd.io created
customresourcedefinition.apiextensions.k8s.io/ocirepositories.source.toolkit.fluxcd.io created
customresourcedefinition.apiextensions.k8s.io/providers.notification.toolkit.fluxcd.io created
customresourcedefinition.apiextensions.k8s.io/receivers.notification.toolkit.fluxcd.io created
serviceaccount/helm-controller created
serviceaccount/kustomize-controller created
serviceaccount/notification-controller created
serviceaccount/source-controller created
clusterrole.rbac.authorization.k8s.io/crd-controller-flux-system created
clusterrole.rbac.authorization.k8s.io/flux-edit-flux-system created
clusterrole.rbac.authorization.k8s.io/flux-view-flux-system created
clusterrolebinding.rbac.authorization.k8s.io/cluster-reconciler-flux-system created
clusterrolebinding.rbac.authorization.k8s.io/crd-controller-flux-system created
service/notification-controller created
service/source-controller created
service/webhook-receiver created
Warning: would violate PodSecurity "restricted:latest": seccompProfile (pod or container "manager" must set securityContext.seccompProfile.type to "RuntimeDefault" or "Localhost")
deployment.apps/helm-controller created
deployment.apps/kustomize-controller created
deployment.apps/notification-controller created
deployment.apps/source-controller created
networkpolicy.networking.k8s.io/allow-egress created
networkpolicy.networking.k8s.io/allow-scraping created
networkpolicy.networking.k8s.io/allow-webhooks created
deployment.apps/helm-controller condition met
deployment.apps/source-controller condition met
deployment.apps/kustomize-controller condition met
deployment.apps/notification-controller condition met