mgr/devicehealth: import _strptime directly

mgr/devicehealth: import _strptime directly

On some machines (in my case, ubuntu 18.04 with 2.7.15+), I see

MGR_MODULE_ERROR Module 'devicehealth' has failed: Failed to import _strptime because the import lockis held by another thread.
    Module 'devicehealth' has failed: Failed to import _strptime because the import lockis held by another thread.

This is apparently an issue with datetime on py2, see

https://www.raspberrypi.org/forums/viewtopic.php?t=166912

Doing the import explicitly is a simple workaround (and resolves the
warning in my test case).

Signed-off-by: Sage Weil <sage@redhat.com>