Use global maven configuration from M2_HOME/conf

Currently SWR uses configuration from user.home directory and from locations pointed by old maven 2 properties (e.g. org.apache.maven.global-settings). Global settings from M2_HOME directory are currently not included. According to http://maven.apache.org/settings.html

There are two locations where a settings.xml file may live:

The Maven install: $M2_HOME/conf/settings.xml
A user's install: ${user.home}/.m2/settings.xml
The former settings.xml are also called global settings, the latter settings.xml are referred to as user settings. If both files exists, their contents gets merged, with the user-specific settings.xml being dominant.

Related code: https://github.com/shrinkwrap/resolver/blob/master/impl-maven/src/main/java/org/jboss/shrinkwrap/resolver/impl/maven/bootstrap/MavenSettingsBuilder.java#L81