Adding flag --pid/-p to pyperf doesn't result as intended

[mpozniak95]

Found during work on filtering PID's (#761). As stated in issue title: Adding flag --pid/-p to pyperf doesn't result as intended. Pyperf seems to ignore this flag and profile everything anyway.

For now instead of filtering PID's and profile only pid's provided by --pid pyperf profile every python process and then filter output to return only pid's provided. This needs to be changed