Move hard-coded values into ns_bucket.hrl

ns_server contains source files with hard-coded values. Some of them are repeatedly used in different files. This ticket tracks moving them into a new ns_bucket.hrl file. Here's some examples:

     {"magma_fragmentation_percentage", [{reload, flush}],

      proplists:get_value(frag_percent, BucketConfig, 50)},

     {"magma_mem_quota_ratio", [{reload, flush}],