Change autoscaler configuration

[petersutter]

What this PR does / why we need it:
When creating a new cluster the autoscaler min size is defaulted to 2. Most users that want to play around do not change the default values and this produces unnecessary costs, hence the default min value is changed to 1.
Also the minimum allowed value for autoscaler min and max value is changed from 1 to 0.

Which issue(s) this PR fixes:
Fixes #131

Special notes for your reviewer:

Release note:

Changes in cluster creation dialog
- The default autoscaler min value changed from 2 to 1
- Allow autoscaler min and max values to be 0

[holgerkoser]

lgtm