[CPDEV-103936] Fix prepare_dns_etc_hosts for work with unavailable nodes

[n549]

Description

When one or more nodes are unavailable, add_node, remove_node procedures have to be run with prepare.dns.etc_hosts or update.etc_hosts task skipped (see PR).
If after that you try to add another node, add_node fails with error like

Temporary failure in name resolution': ['some_node_fqdn']}

Fixes # (issue)
CPDEV-103936

Solution

1. New boolean parameter globals.ignore_unavailable_nodes_for_etchosts_update is added to cluster.yaml with default value False.
   system_prepare_dns_etc_hosts is adjusted to work as previously if globals.ignore_unavailable_nodes_for_etchosts_update= false and update /etc/hosts only at the available nodes if it's set to True.

2. IN is updated with description of new variable.

How to apply

Test Cases

TestCase 1
step 1) Set globals.ignore_unavailable_nodes_for_etchosts_update to true in the cluster.yaml, disable one of the nodes in a cluster and try to add a new node with add_node procedure.
ER: The new node is added successfully
step 2) With the node still disabled and globals.ignore_unavailable_nodes_for_etchosts_update set to true, try to add another new node with add_node procedure.
ER: The new node is added successfully

TestCase 2
When a node is disabled in cluster and globals.ignore_unavailable_nodes_for_etchosts_update is set to true, remove another node with remove_node procedure.
ER: The node is removed successfully.

TestCase 3
Remove globals.ignore_unavailable_nodes_for_etchosts_update variable from cluster.yaml and try to add a node when there is at least one unavailable node.
ER: add_node fails because not all the nodes are available.

TestCase 4
Remove globals.ignore_unavailable_nodes_for_etchosts_update variable from cluster.yaml and try to add a node when all the nodes are available.
ER: add_node finishes successfully.

Checklist

• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• Integration CI passed
• Unit tests. If Yes list of new/changed tests with brief description
• There is no merge conflicts

Unit tests

Indicate new or changed unit tests and what they do, if any.

[ilia1243]

The reason for why it was previously implemented in this way #628 (comment)