Allow setting default container in arquillian.xml containing group definitions

The <container/> element has a default attribute. If default="true", then this container will be started if no arquillian.launcher container name is specified.

The <group/> element is used to define a group of containers to be started. However, the group element has no "default" attribute (see http://www.jboss.org/schema/arquillian/arquillian-1.0.xsd). So, if we have a configuration involving a single container and as group of containers in the same arquillian.xml file, such as:

<arquillian>
<container qualifier="X"/>
<group qualifier="Y"/>
</arquillian>

then we cannot make Y the default.

We should be able to specify that a group configuration is the default to be started among a set of configurations, and so group should have a default attribute as well.

NOTE: when used within a <group/> the "default" attribute of container changes its meaning. According to AK:

"default="true" on a /container means this is the default Configuration used if no arquillian.launch is specified. While for a Group it's the default container to use if no @TargetsContainer is used."