Add instructions for non DHCP networks re install

[phillxnet]

Thanks to forum member greg for highlighting this issue. When installing Rockstor 4 on a network without a DHCP facility the resulting install is inaccessible. It is proposed that we add a note to the Rockstor 4 installer doc to indicate that the preferred approach to resolve this is to use Network Managers command line tool:

nmcli

An example of it's full use would be helpful here so folks can cut and paste more easily and change only either required IP.

Note that as such networks are exclusively the domain of advanced users it is suggested that all we need the the pointer to nmcli as the appropriate tool to use, along with a clean example for ease and clarity.

Please see the following forum link for context: https://forum.rockstor.com/t/static-ip-instalation/7908

[phillxnet]

Linking to a recent change within our installer console output to aid folks in scenarios where no DHCP is found:
rockstor/rockstor-installer#122