hardcode now(), current_timstamp() into the MySQL regex

hardcode now(), current_timstamp() into the MySQL regex

Fixed yet another regression caused by by the DEFAULT rendering changes in
2.0.40 :ticket:`12425`, similar to :ticket:`12488`, this time where using a
CURRENT_TIMESTAMP function with a fractional seconds portion inside a
textual default value would also fail to be recognized as a
non-parenthesized server default.

There's no way to do this other than start hardcoding a list
of MySQL functions that demand that parenthesis are not added around
them, I can think of no other heuristic that will work here.
Suggestions welcome

Fixes: #12648
Change-Id: I75d274b56306089929b369ecfb23604e9d6fa9dd
(cherry picked from commit 8e9f789f1aa0309005e8b7725643b32802e7d214)