Make environment variables available for property replacement in configuration

Copy all the environment variables to the base property "env" in the system property table. Using the base property avoids conflicts with property names. This strategy is typical in build systems.

For example, assume the following environment variable is defined:

JBOSS_HOME=/opt/jboss-as

This environment variable can be referenced from the configuration (arquillian.xml) as follows:

${env.JBOSS_HOME}

A system property may be used to override an environment variable, as follows:

-Denv.JBOSS_HOME=/home/user/opt/jboss-as