allow customising currently hard-coded service hostnames

[tiborsimko]

We have currently hard-coded component hostnames in config.py:

REANA_INFRASTRUCTURE_COMPONENTS_HOSTNAMES = {
    component_name: (
        "{component_prefix}-{component_name}.{namespace}.svc.cluster.local"
    ).format(
        component_prefix=REANA_COMPONENT_PREFIX,
        component_name=component_name,
        namespace=REANA_INFRASTRUCTURE_KUBERNETES_NAMESPACE,
    )
    for component_name in REANA_INFRASTRUCTURE_COMPONENTS
}
"""REANA infrastructure pods hostnames.

This causes troubles for the RIVER-SSL deployment due to using different domain.

(See also the ClusterRole to Role tentative changes here: https://github.com/reanahub/reana/pull/291/files that may or may not be needed.)

Basically, instead of always appending svc.cluster.local, we could make this part customisable via Helm values.

Note that the reana-server component is also concerned:

$ rg svc.cluster.local
• reana_server/reana_admin/cli.py
1080:                f"http://reana-run-session-{workflow_id}.{REANA_RUNTIME_KUBERNETES_NAMESPACE}.svc.cluster.local:8081/{workflow_id}/api/status",