add instructions for air-gapped environments

[v1k0d3n]

per @kmova "could you put in an issue requesting information on how to setup openebs for the air-gapped".

we work with a lot of telecom folks, and are using OpenEBS for 5G applications. these locations are air-gapped with no access to the internet. it would be nice to have some documentation on using OpenEBS in these air-gapped environments. in particular, we ran into an issues with one un-noticed image quay.io/openebs/linux-utils:1.9.0 which was being used for a pv job. it turns out that this is satisfied by the OPENEBS_IO_HELPER_IMAGE variable. some documentation on this could be really helpful for future folks who run into this problem as well.