Turn Velocity template caching on.

As turned out, Templates are always re-read from classpath
causing congestion.

Using SISU Velocity's VelocityConfigurator to configure
enging to Nexus needs (as it was using defaults) set in
VelocityImpl of sisu-velocity.

This NX specific Configurator overrides the defaults from here
https://github.com/sonatype/sisu-velocity/blob/master/src/main/java/org/sonatype/sisu/velocity/internal/VelocityImpl.java#L74

These defaults were used by NX so far.

Verified that caching works (as this is Velocity concern
not Nexus) by debugging. Method (line 294):

org.apache.velocity.runtime.resource.ResourceManagerImpl.getResource(String, int, String)

Verified that template is loaded up once, and subsequent calls
are retrieving it from "globalCache".