Do not hardcode the WildFly Maven Plugin version in image

[jfdenise]

Today the env variable PROVISIONING_MAVEN_PLUGIN_VERSION defines statically the version of the WildFly Maven plugin used to provision the server.
We should let maven discover the latest version. A user would still be able to set a version explicitly thanks to the PROVISIONING_MAVEN_PLUGIN_VERSION env variable.

NOTE: Not specifying a plugin version is not recommended and can give unexpected results according to the maven repositories configuration. This approach shouldn't be followed.

A correct Solution would be to use the
versions plugin that allows to properly upgrade plugin version.