Allow mixed case indices

[costin]

Elasticsearch does not allow indices with non lowercase characters. However this restriction does not apply to aliases. Which can be used for reading and, in case they point to only one index, for writing as well.
ES-Hadoop does case checks when validating a resource however due to the above, this validation might be incorrect and despite being a corner-case (and arguably a not-so-good practice), it should be removed.