[Security Risk] Avoid using shell=true usage wherever subprocess module is used

Subprocess module allows us to execute command on the shell but usage of shell=true poses a security risk where user inputs with "rm -rf" can cause terrible things.

To avoid shell-injection vulnerabilities, subprocess can be used without shell=true, by modifying the way input is passed.

Some of the examples can be found like - https://security.openstack.org/guidelines/dg_avoid-shell-true.html

This Jira is to track the related changes. Please feel free to comment / discuss.