Allow override of default RB server in shipits tool.
Allow override of default RB server in shipits tool.

Reviewed at https://reviews.apache.org/r/19519/

apache/aurora,wfarner/aurora,wfarner/aurora,wfarner/aurora,crashlytics/aurora,mkhutornenko/incubator-aurora,protochron/aurora,thinker0/aurora,wfarner/aurora,rdelval/aurora,rdelval/aurora,rdelval/aurora,mkhutornenko/incubator-aurora,wfarner/aurora,rosmo/aurora,shahankhatch/aurora,thinker0/aurora,kidaa/aurora,medallia/aurora,medallia/aurora,protochron/aurora,apache/aurora,protochron/aurora,mschenck/aurora,shahankhatch/aurora,protochron/aurora,mschenck/aurora,mschenck/aurora,kidaa/aurora,mkhutornenko/incubator-aurora,kidaa/aurora,thinker0/aurora,rosmo/aurora,apache/aurora,shahankhatch/aurora,medallia/aurora,mkhutornenko/incubator-aurora,rosmo/aurora,thinker0/aurora,medallia/aurora,mkhutornenko/incubator-aurora,shahankhatch/aurora,apache/aurora,crashlytics/aurora,mschenck/aurora,thinker0/aurora,mschenck/aurora,kidaa/aurora,rosmo/aurora,mschenck/aurora,rosmo/aurora,rdelval/aurora,kidaa/aurora,rdelval/aurora,kidaa/aurora,rdelval/aurora,crashlytics/aurora,protochron/aurora,crashlytics/aurora,crashlytics/aurora,medallia/aurora,medallia/aurora,thinker0/aurora,shahankhatch/aurora,shahankhatch/aurora,crashlytics/aurora,rosmo/aurora,wfarner/aurora,apache/aurora,protochron/aurora,apache/aurora