GEOMESA-432 Improvements to GeoMesa Tools script 

GEOMESA-432 This PR addresses some usability issues with the geomesa shell script. Now environment variables
for hadoop and accumulo can be optionally set, and individual paths can be overridden by the user.
Also more informative messages are printed when certain variables are not set. 
Instead of this check, do 
```
if [ -z "$GEOMESA_HOME" ]; then
    SOURCE="${BASH_SOURCE[0]}"
    while [ -h "${SOURCE}" ]; do # resolve $SOURCE until the file is no longer a symlink
       bin="$( cd -P "$( dirname "${SOURCE}" )" && pwd )"
       SOURCE="$(readlink "${SOURCE}")"
       [[ "${SOURCE}" != /* ]] && SOURCE="${bin}/${SOURCE}" # if $SOURCE was a relative symlink, we
                                                            # need to resolve it relative to the
                                                            # path where the symlink file was located

    done
    bin="$( cd -P "$( dirname "${SOURCE}" )" && cd ../ && pwd )"
    export GEOMESA_HOME="$bin"
    export PATH=${GEOMESA_HOME}/bin:$PATH
fi

if [ -z "$GEOMESA_LIB" ]; then
    GEOMESA_LIB=${GEOMESA_HOME}/lib
fi
```
And remove `geomesa configure` entirely to address @elahrvivaz's comment