Handle gracefully if no node is master eligible

[tvainika]

About this change: What it does, why it matters

Karapace configuration allows configuring node to not be eligible for master. Handle gracefully ie. read-only mode if all nodes are configured non-eligible for master.

[hackaugusto]

I think it doesn't make sense to have a node that is not master eligible set as master in the metadata.

[tvainika]

I think it doesn't make sense to have a node that is not master eligible set as master in the metadata.

I changed are_we_master logic and renamed is_master_eligible to has_eligible_master, so I think this clarifies the logic

addressed

[hackaugusto]

I changed are_we_master logic and renamed is_master_eligible to has_eligible_master, so I think this clarifies the logic

My bad, I should have been clearer on what I meant by "have a node that is not master eligible set as master in the metadata.". This change is setting the variable master_url even if there is not master. That variable is of type Optional[str], so if its value is None then there should be no master, otherwise it should have the URL set to the current master.

The above means the variable has_eligible_master is unnecessary.

addresses

[hackaugusto]

LGTM