[IZPACK-1194] "contains" condition does not work for checking plain <variable> values

Implements http://jira.codehaus.org/browse/IZPACK-1194:

The "contains" condition, introduced in 5.0, does not work for the following example:

```xml
<condition type="contains" id="mssqlUrlFound">
   <variable>jdbc.url</variable>
   <value>jdbc:jtds:sqlserver</value>
</condition>
```

This affects probably more usecases using plain value checks, because an invalid check causes the condition to make a regex check due to the default byLine="true" instead of comparing plain values. This failed due to an uninitialized regex pattern.