Check if another resource exists when creating an alias

[jsoriano]

When creating an alias, if an index exists with the same name, alias is
not created assuming that it already exists. This can create some
confusion in some cases, as users can see that setup works, but indexes
and aliases are not correctly configured afterwards, leading to field
mapping issues and other kinds of problems.

This change makes beats explictly check if the resource that already
exists is actually an alias, if not, it returns a different failure that
can be seen by the user when running setup.

[urso]

I'm ok with double-checking here, but it would be nice if HasAlias would already report an error if the resource found is no alias.

Maybe we should better use the cat API: https://www.elastic.co/guide/en/elasticsearch/reference/current/cat-alias.html

[jsoriano]

@urso I have added a check also to HasAlias. I am keeping the check also in CreateAlias because it shouldn't succeed if it is not able to create the alias. Let me know what you think.

Regarding using the cat API, I think it is better not to use it. For example it responses with status 200 (and an empty response) when a requested resource doesn't exist, this kind of things would force us to parse the responses.

[jsoriano]

jenkins, test this again please

[jsoriano]

jenkins, test this again please

[jsoriano]

Failing tests are not related, merging this.