Some of the uses for the formerly supported regular expression constraints
were simply to limit values to those in a set of allowed values.
This commit adds support for that kind of simple enumeration constraint.
Regular expression constraints have caused nothing but trouble due to the
lack of a ubiquitous regular expression library. PCRE is *almost*
everywhere, but it has different versions, and different features, and
different bugs, in different places. It is more trouble than it is worth.
So this commit drops support.
It is natural to write "abc" in place of ["set",["abc"]] and vice versa.
I cannot think of a reason not to support this, and it can make reading
and writing OVSDB files and transactions easier, so support it.
A "min" value greater than 1 is problematic for the database. There is no
reasonable way to synthesize a default value for such a column: keys in a
set or map must unique, so the database cannot, say, simply set a set of
3 or more integers to [0, 0, 0].
This should have no effect on the vswitch in practice because it does not
have any columns that require more than one element.
