Change HOST/PORT env to TARGET_HOST / TARGET_PORT (#303)

As discussed here: https://github.com/netobserv/netobserv-ebpf-agent/pull/291#discussion_r1520076865
HOST and PORT are very generic and don't provide the meaning about what
they are for; An uninformed user would probably think this would be for
a server, of for the agent host itself, rather than a target of
flow/packet collectors.