have Flux use the single-replica storage class (longhorn related)

@cristian.manda mentioned that it's been reported (I believe by @lupescu_daniel) that the following case does not work well:

• baremetal + longhorn
• 1 CP node, 1 or more MD node
• some replica storage class is selected for longhorn (not the single-replica one since there are more LH nodes)
• bootstrap:
  • CP nodes comes up
  • flux starts to be installed, but Flux PV can't use the storage class because only a single node is up
• this blocks the deployment

A solution would be to have Flux always use the single-replica longhorn class.

/cc @cristian.manda @feleouet @lupescu_daniel