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.

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".
Applied comments.

Changes:
* added UT verifying that things are set
* simplified NexusVelocityConfigurator, no more dev/prod differentiation
Javadoc updated
Merge pull request #830 from sonatype/velocity-caching

[REVIEW] Turn Velocity template caching on.