(maint) use matching for cows dist and arch instead of splitting based on dashes

This commit changes the way we set the DIST and ARCH environment based on the cow name.  Instead of splitting based on "-", we use matching for the DIST and ARCH.  This allows us to have cows with a dash in the DIST name ( such as CumulusLinux-2.2 ).
I think we should avoid hard-coding the arches here. Would it be sufficient to do
`/base-(.*)-(.*).cow/.match(cow)[1]` ?