NEXUS-6090: Getting rid of SISU Velocity.

That introduced component based configuration
for VE. Using plain provider pattern, as the
SISU Velocity was basically doing that same,
except in "plx way".

Changes:
* banned sisu-velocity from build
* upped goodies and mailer dependencies (that has needed changes too)
* removed SISU Velocity related code from Nx core
* introduced Velocityengine provider
* adapted restlet1x internals
* adapted VelocityContentGenerator

Related
https://github.com/sonatype/sisu-goodies/pull/19
https://github.com/sonatype/sisu-mailer/pull/4

Issue
https://issues.sonatype.org/browse/NEXUS-6090

CI
https://bamboo.zion.sonatype.com/browse/NX-OSSF70