Add a build argument for distro

This is to be picked up in install.sh (and potentially other scripts if
needed). The scripts will attempt to determine this automatically using
/etc/*release in the case this is not specified (default behaviour).

Change-Id: I6c09e5ea5249be946f5f4db25dfc3877795912a5

centos/Dockerfile

@@ -9,6 +9,7 @@ ARG PROJECT_REF=master
 ARG SCRIPTS_REPO=https://github.com/openstack/loci
 ARG SCRIPTS_REF=master
 ARG OVERRIDE=override
+ARG DISTRO
 
 ADD $OVERRIDE /
 

debian/Dockerfile

@@ -9,6 +9,7 @@ ARG PROJECT_REF=master
 ARG SCRIPTS_REPO=https://github.com/openstack/loci
 ARG SCRIPTS_REF=master
 ARG OVERRIDE=override
+ARG DISTRO
 
 ADD $OVERRIDE /
 

ubuntu/Dockerfile

@@ -9,6 +9,7 @@ ARG PROJECT_REF=master
 ARG SCRIPTS_REPO=https://github.com/openstack/loci
 ARG SCRIPTS_REF=master
 ARG OVERRIDE=override
+ARG DISTRO
 
 ADD $OVERRIDE /