Stay with Softlayer library <=5.7.2

According to https://github.com/softlayer/softlayer-python  we have to stay Softlayer library <=5.7.2 since we are using python 3.5.2
This is to avoid
AttributeError: module 'os' has no attribute 'PathLike'
Details ---> gitpython-developers/GitPython#1219

Author: donacarr

ci/xenial/tasks/import-stemcell-bionic.sh

@@ -6,7 +6,7 @@ export CANDIDATE_BUILD_NUMBER=$( cat version/number | sed 's/\.0$//;s/\.0$//' )
 echo -e "\n[INFO] Install tools..."
 apt-get update
 apt-get -y install python3-pip
-pip3 install 'SoftLayer>=5.6.0'
+pip3 install 'SoftLayer>=5.6.0,<=5.7.2'
 pip3 install click==7.0
 export LC_ALL=C.UTF-8
 export LANG=C.UTF-8