Allow alternative path to be specified for Hadoop client JARs

**Is your feature request related to a problem? Please describe.**
It would be nice to have a baked-in way to specify an alternate path for the location of Hadoop client libraries. By default, Accumulo attempts to add Hadoop jars from `$HADOOP_HOME/share/hadoop/client`. This works when the Hadoop installation follows the tarball-style pathing, but not when jars are unpacked to a different location, such as when Hadoop is installed via RPMs.

**Describe the solution you'd like**
I would like accumulo-env.sh to define an environment variable that by default points to `$HADOOP_HOME/share/hadoop/client` if not already defined, and for the CLASSPATH to add Hadoop jars using this new variable.

**Describe alternatives you've considered**
Of course, I can edit the accumulo-env.sh on my own installation, but I believe having this feature baked-in would help make Accumulo setup more flexible.

Start in accumulo-env.sh and inspect how the current $HADOOP_HOME/share/hadoop/client path is used when building CLASSPATH. Add the requested overridable environment variable with that default, then verify that CLASSPATH uses it for Hadoop client JARs and supports an alternate installation path.

Written by the indexing model from the issue text.