ILM allows a policy name that generates invalid snapshot name

[dakrone]

We allow characters in the ID of an ILM policy that will result in an invalid snapshot name.

For example, an ILM policy with the name mypolicy-* can be created, however, the generation of the snapshot name in this line:

elasticsearch/x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/ilm/GenerateSnapshotNameStep.java

Line 81 in befb6fa

String snapshotNamePrefix = ("<{now/d}-" + index.getName() + "-" + policyName + ">").toLowerCase(Locale.ROOT);

Results in the mypolicy-* being part of the snapshot name, which the snapshot service then disallows. We should strip these invalid characters from the snapshot name.

[elasticsearchmachine]

Pinging @elastic/es-data-management (Team:Data Management)