AURORA-108: make set_quota in aurora_admin require explicit units

set_quota now requies a specific unit (M or G) as input.

This means that arguments to set_quote should resemble the following:
    set_quota(<cluster>, <role>, <cpu>, '50M', '20G')
    set_quota(<cluster>, <role>, <cpu>, '50G', '20G')

Testing Done:
Passes local tests. I could not find a test that tests the quota for the cli client so this patch does not contain an dedicated test case.

Bugs closed: AURORA-136

Reviewed at https://reviews.apache.org/r/17581/