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:

https://github.com/elastic/elasticsearch/blob/befb6faafc9fca327fd3cea2d0ab328a521c4e65/x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/ilm/GenerateSnapshotNameStep.java#L81

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.
x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/ilm/GenerateSnapshotNameStep.java
public static final String NAME = "generate-snapshot-name";
    private static final Logger logger = LogManager.getLogger(GenerateSnapshotNameStep.class);
    private final String snapshotRepository;