Edit index set "rotation strategy" changes also "Max number of indices"

**Change field "rotation strategy" from "Index Time" to "Archive" changes also field "Max number of indices" from 5 back to the default value of 20.**

## Expected Behavior
Field "Max number of indices" should keep it's value.

## Current Behavior
Field "Max number of indices" changes from 5 back to the default value of 20.

## Possible Solution
Fix bug.

## Steps to Reproduce (for bugs)
1. create index set with "rotation strategy" equals "Index Time" and "Max number of indices" equals 5.
2. Edit index set
2. Change "rotation strategy" to "Archive".
3. Look for changed "Max number of indices" field.

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Versions:
- Graylog 2.2.3
- elasticsearch 2.4.4
- mongoDB 3.2.1
- Linux SLES 11
- Browser Firefox ESR 45.9.0
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