Add validator to parse the time unit set in Metastore hive.repl.cm.retain

In HiveConf.java the default time unit for hive.repl.cm.retain is DAYS.

But in metastoreConf the default unit is HOUR. If one sets '10' in both, the actual values are going to be different.

In this jira, I add a validator to parse values such as '10d' which will be interpreted correctly in both.