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/test/java/org/elasticsearch/xpack/core/ilm/GenerateSnapshotNameStepTests.java
protected GenerateSnapshotNameStep copyInstance(GenerateSnapshotNameStep instance) {
        return new GenerateSnapshotNameStep(instance.getKey(), instance.getNextStepKey(), instance.getSnapshotRepository());
    }
    public void testPerformAction() {
        String indexName = randomAlphaOfLength(10);
        String policyName = "test-ilm-policy";
        final IndexMetadata indexMetadata = IndexMetadata.builder(indexName)
            .settings(settings(Version.CURRENT).put(LifecycleSettings.LIFECYCLE_NAME, policyName))
            .numberOfShards(randomIntBetween(1, 5))
            .numberOfReplicas(randomIntBetween(0, 5))
            .build();
        GenerateSnapshotNameStep generateSnapshotNameStep = createRandomInstance();
        
        RepositoryMetadata repo = new RepositoryMetadata(generateSnapshotNameStep.getSnapshotRepository(), "fs", Settings.EMPTY);
        ClusterState clusterState = ClusterState.builder(emptyClusterState())
            .metadata(
                Metadata.builder()
                    .put(indexMetadata, false)
                    .putCustom(RepositoriesMetadata.TYPE, new RepositoriesMetadata(Collections.singletonList(repo)))
                    .build()
            )
            .build();
        ClusterState newClusterState;
        
        newClusterState = generateSnapshotNameStep.performAction(indexMetadata.getIndex(), clusterState);
        LifecycleExecutionState executionState = newClusterState.metadata().index(indexName).getLifecycleExecutionState();
        assertThat(executionState.snapshotIndexName(), is(indexName));
        assertThat(
            "the " + GenerateSnapshotNameStep.NAME + " step must generate a snapshot name",
            executionState.snapshotName(),