[ML] Add remote cluster license check to Validate Job

[LucaWintergerst]

Elasticsearch version (bin/elasticsearch --version): 6.5.1

Plugins installed: []

JVM version (java -version): 1.8.0_102

OS version (uname -a if on a Unix-like system): Darwin

Description of the problem including expected versus actual behavior:

When creating an ML job, the Validate function allows to run a set of pre-flight checks to ensure that a job will work as expected.
However, this check does not verify that a remote cluster has a sufficient License

Steps to reproduce:

1. launch 1 node, cluster_a with platinum/trial license
2. launch 1 node, cluster_b with a basic license
3. add CCS settings to cluster_a
4. add ccs index pattern to cluster_a
5. create advanced job using ccs pattern, click on validate -> all green
6. attempt to start the job -> it won't run

[elasticmachine]

Pinging @elastic/ml-core

[LucaWintergerst]

had a chat with @dimitris-athanasiou and @droberts195
This is something that has to be done in Kibana, but we'll likely need some additional backend enhancements to offer that information to Kibana. It's therefore not a low hanging fruit and will take more work