[3.0] Generate the /etc/hosts file with a custom state

[inercia]

(Backport of #616)

Generate the /etc/hosts file from a state, merging our entries with previously found entries.

• first time we runn this state, we copy /etc/hosts to /etc/caasp/hosts
• after that, /etc/hosts is written as we have done so far (with entries for master, workers, etc) and merged with all the entries found in /etc/caasp/hosts.

(Users are instructed with a comment in /etc/hosts to add their custom entries in /etc/caasp/hosts)

See https://bugzilla.suse.com/show_bug.cgi?id=1098334

bsc#1098334

[mjura]

I have re-ran CI, we have to merge this PR, it is needed also to fix http://bugzilla.suse.com/show_bug.cgi?id=1097294

[bergmannf]

I currently don't have enough RAM free on my machine to try this backport, as I am already working on another update-related bug.

Can someone verify that this backport does not add an admin alias to one of the other nodes in the /etc/hosts file - I remember seeing this before.

[mjura]

CI finished successfully, we need this change!