Performance improvement: Replace all applicable UDFs with PDFs

[ConstrainedRandomVar]

The factory default rules still seem to have extensive usage of UDFs through Python functions. These should be converted to corresponding PDFs wherever supported.

• All decimal conversions, division, subtraction, math functions should be converted to eval
• String concatenation can also be done through concatenate
• Rate of change calculation can be handled through rate-of-change
• Value difference calculation can be handle through value-difference

For example:

• chassis.power/check-pem-power-usage: used_percentage.py
• kernel.ifstate/check-too-many-dead-ifstates: get-dead-alive-ratio.py
  ...

[vvikramb]

Eval function has replaced the UDF's wherever possible.