Make system properties accessible in configuration customization

[MartinLeidig]

The system properties should be accessible during configuration customization, e.g. to set paths relative to catalinabase.

[MartinLeidig]

Added system properties to buildtime properties. Access works like for the pre-defined properties (mode, machine, fqdn, uid), e.g.

<cus:when test="$user.name='foo'">

${catalina.base}/bar

</cus:when>

[MartinLeidig]

to clarify: didn't add system properties to property file during build time, only made them accessible like buildtime.properties at runtime/during customization