schema: secret: Relax requirements for usage name

schema: secret: Relax requirements for usage name

There's plenty of existing documentation [1] which shows as example a
name which contains a space and a dot ('client.admin secret') as ceph
usage name.

Use a more relaxed type in the RNG schema since the usage name is
actually just a string used to look up the secret.

[1]:
https://docs.ceph.com/en/latest/rbd/libvirt/#configuring-the-vm
https://documentation.suse.com/ses/6/html/ses-all/cha-ceph-libvirt.html#ceph-libvirt-cfg-vm
Libvirt docs were correct though:
https://libvirt.org/formatsecret.html#CephUsageType

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1689168

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>