docs: fix typo in INSTALL.rst

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>
base: eval is evil

* Using ast.literal_value instead of eval to parse user provided values.
  Safer and more reliable (bool("False") -> True)

* Removing the checks that the new type corresponds with the known type
  for the variables.

* Adding some tests.

Signed-off-by: Yoan Blanc <yoan.blanc@cern.ch>