Fix support for puppetlabs-stdlib<4.2.0

Support for non-array value in the function concat() was added
in puppetlabs-stdlib 4.2.0.

This change adds a fix to support puppetlabs-stdlib<4.2.0
by converting $host to an array.

Change-Id: Ib8e4c7a5f788d1e0bd7440bc93b560df8e39fcf4