On CREATE TABLE, consider skipping validation of subpartitions.
This is just like commit
14f67a8ee282ebc0de78e773fbd597f460ab4a54, but
for CREATE PARTITION rather than ATTACH PARTITION.
Jeevan Ladhe, with test case changes by me.
Discussion: http://postgr.es/m/CAOgcT0MWwG8WBw8frFMtRYHAgDD=tpt6U7WcsO_L2k0KYpm4Jg@mail.gmail.com