Add fallback for Certificate.not_valid_before/after_utc #495

Job	Run time
Waiting for build / Waiting For Build	7m 37s
KRA clone with HSM / Test	9m 43s
KRA clone with replicated DS / Test	7m 4s
KRA clone with shared DS / Test	5m 53s
KRA clone / Test	9m 12s
	39m 29s

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