DP-426 Parametrized rtmax,wtmax,and dtperf(HDFS-6080)

Parametrized rtmax,wtmax,and dtperf for HDFS-6080. In hdfs-site.xml,
this can be enhanced with dfs.nfs3.rtmax, dfs.nfs3.wtmax, and
dfs.nfs3.dtmax respectively.
Merge pull request #46 from VertiPub/aws-HDFS-6080-DP-426-parametrized-rtmax

DP-426 Parametrized rtmax,wtmax,and dtperf(HDFS-6080)