Ability to use proxy in MavenUtil

WildFly Core testsuite can be run on pure-IPv6 machines. Proxy could be used in that case.

But some tests use MavenUtil class for download some dependencies. And this class can not use proxy server. So these tests fail.

This MavenUtil should check "proxyHost" and "proxyPort" environment variables. If they are set, MavenUtil class should set proxy. If these variables are not set, proxy is not used. These variables are used by maven too.