snapshot policy should allow specifying indices

[wyllys66]

Currently, (release 1.12.0), it appears that we cannot specify what indexes to include in a snapshot when the snapshot is triggered by a policy. The policy action for "snapshot" only includes the repository and the name of the snapshot. It should allow for additional parameters related to snapshot operations.

[wyllys66]

While it is possible to create index_templates that assign a snapshot policy to a list of matching indices, the problem with that approach is that it results in a separate snapshot for each index. It would be so much better if there was a single snapshot that included the matching indices. This is pretty significant.

[lav-kush]

Currently, it appears that we cannot specify dynamic snapshot name in ISM policy. The policy action for "snapshot" only includes the repository and the static name of the snapshot, and it creates separate snapshot for each indices with name -. It should allow to pass dynamic snapshot name say for each index something like ctx.index var as used in message template.