helm: administrators to choose allowed workspace options

[tiborsimko]

When deploying REANA, users can run workflows on several workspaces. Right now, any POSIX filesystem mounted on cluster nodes is supported.

In theory, users could ask for any such mounted filesystem:

inputs:
  options:
    workspace_root_path: /usr/local/share

In practice, the administrators should limit what is allowed to use.

For example, empty value is allowed and it will use the default shared filesystem (/var/reana/users/aaaa/workflows), or /myshareddisk/ is allowed, etc.

This issue is about allowing REANA cluster administrators to easily choose the available prefix values via Helm. We need a new variable for helm/reana/README.md named for example workspaces.paths that would be a list of available path prefixes, the first being the default.

The cluster would then mount automatically these values to all the necessary pods.

P.S. The users would choose which one to use via reana.yaml, see another issue reanahub/reana-client#545