Use Stringext.has_substr instead of needing Re package.

A fix-up after commit f0c11881ccfeb0d0cd0ed5e263d7fd3185f23319
which brought in a fix from trunk, this commit removes the use
of the Re package.
(That package is not present on the Clearwater-derived branch.)

Signed-off-by: Thomas Sanders <thomas.sanders@citrix.com>