feat: Support multiple reservations (super-slicing sub-block targeting)

[jamOne-]

Description

Allow using multiple reservation during cluster creation with --reservation=res1,res2 (comma-separated).

• In case of multiple reservations, one reservation will be used for exactly one nodepool (number of reservations has to match the number of nodepools to be created).
• In case of one reservation, the same reservation will be used for every nodepool (as it was before the change).

Issue

http://b/474095154 -- Allow for sub-block targeting for super-slicing clusters, which as a byproduct allows for using multiple reservations during cluster creation.

Testing

• Unit tests
• Manually tested multiple reservations for superslicing