Support timezone selection for monitor schedules set "By interval"

Is your feature request related to a problem? Please describe.
When scheduling a Monitor, I find it inconsistent that the 'By interval' type does not expose a Timezone field, just like the daily, weekly, monthly and custom cron expression do. Connected to that, on the monitor creation API (POST _opendistro/_alerting/monitors) if 'period' type is selected under 'schedule' then "timezone" field cannot be specified (I guess it's just ignored).

Describe the solution you'd like
Both UI and API should support the Timezone field.

• UI should expose the Timezone field on "By interval" schedule type, just like rest types.
• API should allow setting "timezone": "America/Los_Angeles" under 'schedule' object for 'period' type, just like rest types.

Describe alternatives you've considered
An work-around is to use 'custom cron expression', which of course isn't as user-friendly as 'by interval', but certainly does the job.

Additional context
I'm using OpenDistro 1.13.2. I understand that this is a minor thing, especially since there is an easy and valid alternative. I'm just suggesting it because it creates inconsistency on user experience. Personally when I came up with timezone discrepancy in my alarms (configured on UI) I didn't even think that setting Timezone is possible with schedule types other than 'By interval'. That's a problem especially because 'By interval' is selected by default.

Start at the monitor creation API, POST _opendistro/_alerting/monitors, and trace how a period schedule handles its timezone field. Then locate the UI implementation for the "By interval" schedule type. Done means both UI and API accept and apply a timezone for interval schedules, matching the other schedule types.