Fix CVE-2019-15597 - Implement new DF command instead of library function

[nadavMiz]

Explain the changes

1. implemented a local function to replace the unmaintained node-df library. this function is based on the node-df implementation with changes to fit our code better (https://github.com/adriano-di-giovanni/node-df).
2. add '' before all special characters in the file name so bash will read them as part of the file name itself. this is to prevent the code injection suggested in CVE-2019-15597

Issues: Fixed #xxx / Gap #xxx

1. https://bugzilla.redhat.com/show_bug.cgi?id=2124534

Testing Instructions:

1. df is used mainly during node creation, so create a new backing store and validate that it was created successfully

• Doc added/updated
• Tests added

[nimrod-becker]

lets add a unit test

[nadavMiz]

lets add a unit test

since df checks the available space on the whole file system, which I have no control over, I can't test the validity of the actual values. so instead I just check that the command returns a valid response and that no errors are thrown

[nadavMiz]

I have added try catch to df calls with file name since now the function throws if the file doesn't exist and does not just return a null value

[liranmauda]

LGTM