Add sensor reading timer

Added timer for reading sensor at regular interval defined for each
sensor in configuration file.

Tested: Verified this by running following busctrl command and checked
value as it is changing regularly.
busctl introspect --no-page xyz.openbmc_project.HealthMon
/xyz/openbmc_project/sensors/utilization/CPU
busctl introspect --no-page xyz.openbmc_project.HealthMon
/xyz/openbmc_project/sensors/utilization/Memory

Signed-off-by: Vijay Khemka <vijaykhemka@fb.com>
Change-Id: I7da085e24b68c0b9f9bf1c13c567024c69f65d70
2 files changed