Update rally-* index template settings to default to Elasticsearch defaults

[b-deam]

In #1309 we added
configurable shard and replica shard counts to rally.ini.
In that commit, the default number of replicas was set to 0,
which is trappy given that Elasticsearch defaults to 1 replica.

This commit changes the default behaviour to add 1 replica to
the rally-* indices by default.

See comment: #1312 (comment)

[dliappis]

As discussed offline let's change the logic to default to None and only attempt to override the number of shards and number of replicas if they've been explicitly set.
Additionally we'll need to remove the explicit "number_of_shards":1 from the three template files, e.g. here. With these changes implemented, metric stores with a default rally.ini will use the Elasticsearch index defaults for the rally-* indices (i.e. 1p/1r) and allow the user to override any of the two settings as needed.

[b-deam]

As discussed, I've reworked the PR to meet the new requirements we discussed above ^^

The number of primary and replica shards are still configurable via rally.ini, but instead of explicitly settings defaults via rally, we fallback and allow Elasticsearch to set the defaults. We 'inject' these values into the respective rally metrics/races/results templates if a user has specified either or both datastore.number_of_shards/datastore.number_of_replicas.

In addition, we also remove the hardcoded refresh_interval: 5s setting to allow us to take advantage of Elasticsearch's default refresh behaviour:

index.refresh_interval
How often to perform a refresh operation, which makes recent changes to the index visible to search. Defaults to 1s. Can be set to -1 to disable refresh. If this setting is not explicitly set, shards that haven’t seen search traffic for at least index.search.idle.after seconds will not receive background refreshes until they receive a search request. Searches that hit an idle shard where a refresh is pending will wait for the next background refresh (within 1s). This behavior aims to automatically optimize bulk indexing in the default case when no searches are performed. In order to opt out of this behavior an explicit value of 1s should set as the refresh interval.

[dliappis]

Looks good, left a comment about making the docs about the default value more factual.

[b-deam]

Alrighty - I think I've addressed the comments and added a couple new tests. Can you take a look?

[dliappis]

This looks good, but we need to print the right ini file, honoring custom ini file invocations too.

[dliappis]

There's a small bug left.

[dliappis]

LGTM