Allow overriding akka-actors.version from parameter.

Allow overriding akka-actors.version from parameter.

This allows setting a new akka-actors using a jenkins parameter.

The commit also fixes a problem when SCALA_VER_BASE is defined.
In this case, versions.properties was not parsed, and
deriveModuleVersions would not work as expected: we still want to
read module versions from versions.properties.