Allow reading config parameters from environment variables

[bpkroth]

Rather than generating a new config for each experiment, it can sometimes be useful to templatize a config with some external scripts.

Even more useful would be to support templated configs via benchbase directly, reading the values out of environment variables, or else substituting with defaults.

For instance:

<?xml version="1.0"?>
<parameters>
    <template>true</template>

    <!-- Connection details -->
    <type>POSTGRES</type>
    <driver>org.postgresql.Driver</driver>
    <url>jdbc:postgresql://${BENCHBASE_SERVER_ADDRESS:localhost}:5432/benchbase?sslmode=disable&amp;ApplicationName=tpcc&amp;reWriteBatchedInserts=true</url>
    <username>${BENCHBASE_USERNAME:admin}</username>
    <password>${BENCHBASE_PASSWORD:password}</password>
    <reconnectOnConnectionFailure>true</reconnectOnConnectionFailure>
    <isolation>TRANSACTION_SERIALIZABLE</isolation>
    <batchsize>128</batchsize>

    <!-- Scale factor is the number of warehouses in TPCC -->
    <scalefactor>${BENCHBASE_SCALEFACTOR:1}</scalefactor>

    <!-- The workload -->
    <terminals>1</terminals>
    <works>
        <work>
            <time>${BENCHBASE_TIME:60}</time>
            <rate>${BENCHBASE_RATE:10000}</rate>
        </work>
    </works>

    <!-- ... -->
</parameters>