Externalise basic host BOOTSTRAP script into bash script in crw repo for reuse

Tech debt.

In a number of jenkinsfiles now, we define a nearly-identical BOOTSTRAP bash script that sets up ssh config, known hosts keys and kerberos login.

We should extract this generic stuff that ALL the jenkinsfiles need into a script that can simply be curl'd and run.

Some of these steps should be implemented globally at the Jenkins main level or in the child nodes themselves.

See https://gitlab.cee.redhat.com/codeready-workspaces/cloud-slaves.git

Affected jobs:

• ./codeready-workspaces/dependencies/che-devfile-registry/Jenkinsfile
• ./codeready-workspaces/dependencies/che-jwtproxy/Jenkinsfile
• ./codeready-workspaces/dependencies/che-machine-exec/Jenkinsfile
• ./codeready-workspaces/dependencies/che-pluginbroker/Jenkinsfile
• ./codeready-workspaces/dependencies/che-plugin-registry/Jenkinsfile
• ./codeready-workspaces/dependencies/kubernetes-image-puller/Jenkinsfile
• ./codeready-workspaces-operator/operator.Jenkinsfile
• ./codeready-workspaces-operator/operator-metadata.Jenkinsfile
• ./codeready-workspaces-theia/Jenkinsfile

Other jobs doing the same setup/bootstrapping steps (that should be converted to Jenkinsfiles):

• ./CRW_CI/view/Pipelines/job/get-sources-rhpkg-container-build/config.xml
• ./CRW_CI/view/Pipelines/job/push-rebuilt-container-to-quay/config.xml
• ./CRW_CI/view/Releng/job/crw_sync-github-to-pkgs.devel/config.xml

Reason for this work:

a) [Major] tech debt - less duplicate code
b) [Blocker] when two or more jobs write to ~/.ssh/known_hosts file at the same time, one or more may fail to read the file and therefore the build fails