Update maven plugin artifactId to soapui-maven-plugin

The plugin artifactId currently does not follow conventions for plugins whose  groupId is not org.apache.maven.plugins. 
We also get the following warning when building the  plugin
```
[ERROR]`Artifact Ids of the format maven-___-plugin are reserved for
plugins in the Group Id org.apache.maven.plugins
Please change your artifactId to the format ___-maven-plugin
In the future this error will break the build.
```

The module of the plugin has been renamed during maven3 build migration; it is now soapui-maven-plugin. As the groupId has also changed, I think it is the right time to update the artifactId ot soapui-maven-plugin. User of the plugin will be required to modify the plugin coordinates only one.

You will find the change in this PR