[BUG] Rollover alias should not be required to attach a policy to a data stream

[phestecAMZN]

What is the bug?
Documentation indicates that when attaching a policy containing a rollover to a data stream, you do not need to provide a rollover alias. Currently, not providing one creates a validation error and prevents the policy from being attached.

**How can one reproduce the bug?**r
Steps to reproduce the behavior:

Step 1:
Create data stream with the example named 'logs-example'

PUT _index_template/logs-template
{
"index_patterns": [
"my-data-stream",
"logs-*"
],
"data_stream": {},
"priority": 100
}
POST logs-example/_doc
{
"message": "login attempt failed",
"@timestamp": "2013-03-01T00:00:00"
}

Step 2:
Create ISM policy

PUT _plugins/_ism/policies/policy_1
{
"policy": {
"description": "ingesting logs",
"default_state": "ingest",
"states": [
{
"name": "ingest",
"actions": [
{
"rollover": {
"min_doc_count": 2
}
}
],
"transitions": [
{
"state_name": "search"
}
]
},
{
"name": "search",
"actions": [],
"transitions": [
{
"state_name": "delete",
"conditions": {
"min_index_age": "5m"
}
}
]
},
{
"name": "delete",
"actions": [
{
"delete": {}
}
],
"transitions": []
}
],
"ism_template": {
"index_patterns": ["logs-*"],
"priority": 100
}
}
}
Step 3:
Use the interface to attach the policy 'policy_1' to 'logs-example' you will be prompted to enter a 'Rollover alias'. It will not let you attach using the interface if you don't enter a 'Rollover alias' which in this case it should be optional.

What is the expected behavior?
Should be able to attach policy with or without a rollover alias

What is your host/environment?

• OS: [e.g. iOS]
• Version [e.g. 22]
• Plugins

Do you have any screenshots?

Do you have any additional context?
Add any other context about the problem.