ZenDiscovery constructor needs ElectMasterService instance

Introduced in https://github.com/elasticsearch/elasticsearch/pull/7336 (elasticsearch 1.4 and 2.0), we need to change AzureDiscovery constructor.

Closes #34.
Replace Strings.startsWithIgnoreCase()

`Strings.startsWithIgnoreCase()` has been removed in elasticsearch 1.4 ans master. See https://github.com/elasticsearch/elasticsearch/pull/7428

Closes #35.