ACCUMULO-2944 Fixes harded coded Sun JVM in config and env

ACCUMULO-2944 add support for multiple java vendors in conf & scripts

Support for multiple java vendors is added by getting the
bootstrap_config.sh to generate correct configuration depending on 
specified java vendor. Following details depend on Java vendor

* GC settings
* Exclude/Include JAXP implementation
* Default NRG provider

 
A new question is being asked in the bootstrap_config.sh for Java 
vendor. This is then used to set GC settings, exclude/include JAXP 
implementation in ACCUMULO_OPTS. Also, default NRG provider is being 
injected as system property on command line since IBM JVM does not 
have SUN registered as default provider.
where does this exported variable actually get used?