ARROW-2922: [Release] Make python command name customizable
ARROW-2922: [Release] Make python command name customizable

We can use python3 command by "PYTHON=python3 dev/release/00-prepare.sh".

python3 is used for Python 3 on Debian GNU/Linux.

Author: Kouhei Sutou <kou@clear-code.com>

Closes #2332 from kou/release-make-python-customizable and squashes the following commits:

e3fa1f5c <Kouhei Sutou>  Make python command name customizable

laurentgo/arrow,majetideepak/arrow,icexelloss/arrow,majetideepak/arrow,cpcloud/arrow,icexelloss/arrow,laurentgo/arrow,majetideepak/arrow,tebeka/arrow,pcmoritz/arrow,pcmoritz/arrow,tebeka/arrow,tebeka/arrow,pcmoritz/arrow,cpcloud/arrow,icexelloss/arrow,laurentgo/arrow,xhochy/arrow,renesugar/arrow,xhochy/arrow,majetideepak/arrow,laurentgo/arrow,cpcloud/arrow,apache/arrow,icexelloss/arrow,itaiin/arrow,kou/arrow,apache/arrow,wesm/arrow,pcmoritz/arrow,apache/arrow,renesugar/arrow,icexelloss/arrow,xhochy/arrow,xhochy/arrow,itaiin/arrow,laurentgo/arrow,cpcloud/arrow,tebeka/arrow,icexelloss/arrow,kou/arrow,renesugar/arrow,cpcloud/arrow,majetideepak/arrow,itaiin/arrow,apache/arrow,cpcloud/arrow,renesugar/arrow,xhochy/arrow,xhochy/arrow,cpcloud/arrow,pcmoritz/arrow,renesugar/arrow,pcmoritz/arrow,itaiin/arrow,pcmoritz/arrow,apache/arrow,pcmoritz/arrow,renesugar/arrow,icexelloss/arrow,majetideepak/arrow,xhochy/arrow,apache/arrow,apache/arrow,cpcloud/arrow,laurentgo/arrow,apache/arrow,kou/arrow,wesm/arrow,icexelloss/arrow,apache/arrow,itaiin/arrow,wesm/arrow,cpcloud/arrow,majetideepak/arrow,xhochy/arrow,renesugar/arrow,pcmoritz/arrow,xhochy/arrow,itaiin/arrow,kou/arrow,majetideepak/arrow,tebeka/arrow,wesm/arrow,laurentgo/arrow,wesm/arrow,tebeka/arrow,cpcloud/arrow,kou/arrow,tebeka/arrow,laurentgo/arrow,kou/arrow,apache/arrow,renesugar/arrow,xhochy/arrow,renesugar/arrow,renesugar/arrow,laurentgo/arrow,icexelloss/arrow,renesugar/arrow,itaiin/arrow,wesm/arrow,itaiin/arrow,cpcloud/arrow,pcmoritz/arrow,renesugar/arrow,laurentgo/arrow,wesm/arrow,itaiin/arrow,icexelloss/arrow,kou/arrow,kou/arrow,kou/arrow,wesm/arrow,itaiin/arrow,itaiin/arrow,pcmoritz/arrow,itaiin/arrow,xhochy/arrow,icexelloss/arrow,kou/arrow,wesm/arrow,icexelloss/arrow,majetideepak/arrow,kou/arrow,kou/arrow,pcmoritz/arrow,tebeka/arrow,pcmoritz/arrow,majetideepak/arrow,majetideepak/arrow,cpcloud/arrow,laurentgo/arrow,kou/arrow,renesugar/arrow,renesugar/arrow,laurentgo/arrow,wesm/arrow,wesm/arrow,wesm/arrow,xhochy/arrow,icexelloss/arrow,apache/arrow,majetideepak/arrow,pcmoritz/arrow,itaiin/arrow,renesugar/arrow,apache/arrow,kou/arrow,pcmoritz/arrow,xhochy/arrow,cpcloud/arrow,cpcloud/arrow,wesm/arrow,apache/arrow,itaiin/arrow,xhochy/arrow,apache/arrow,laurentgo/arrow,majetideepak/arrow,laurentgo/arrow,majetideepak/arrow,wesm/arrow,tebeka/arrow,xhochy/arrow,laurentgo/arrow,icexelloss/arrow,tebeka/arrow,majetideepak/arrow,itaiin/arrow