ftests: Cleanup pid logic in functional tests

ftests: Cleanup pid logic in functional tests

PIDs were inconsistently being managed in the functional tests.  Some
functions treated them as `int` and in others they were `str`.  Modify
all functions that deal with PIDs to treat them as `int` or a list of
`int`s.

Reviewed-by: Kamalesh Babulal <kamalesh.babulal@oracle.com>
Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>