Dockerfile: update Trex version, remove /etc/environment
Dockerfile: update Trex version, remove /etc/environment

We need Trex v2.20 to workaround Python 3.5 pyzqm import error

Also /etc/enviroment doens't work with Docker so we need
to set all vars using ENV A="B"

Change-Id: I5ffc2043d6e0c6f37f579bea5a37c33d85a7ed3f
Signed-off-by: Ross Brattain <ddebe0db9486ace8e4376a8be8a0aaa0090ac511@intel.com>

orestkreminskyi/taf,orestkreminskyi/taf,orestkreminskyi/taf,taf3/taf,taf3/taf,taf3/taf