feature: replicaGroup

[UnamedRus]

Ability to define groups of replicas in easy way, so they can share specific podTemplate:

For example replicas 1 & 2 should use regular ec2 instances and replicas 3+ should use ephemeral ec2.

Right now, it will require you to define podTemplate for each "overloaded" replica, which will produce a lot of noise in chi.