nuageinit: validate set-name to prevent shell injection in variable names

Shell variable names cannot be safely quoted with shell_escape() —
only alphanumeric characters are valid. Add validation that set-name
only matches [a-zA-Z0-9]+; invalid values are rejected with a
warning and the rename is skipped entirely.

(cherry picked from commit 13fb6dbc738f4ba30e78a8fb21efa1382c520d33)