In python `is` checks object identity, not content:

    In [1]: a = 'hello'

    In [2]: a += ' world'

    In [3]: b = 'hello world'

    In [4]: id(a)
    Out[4]: 139735676540976

    In [5]: id(b)
    Out[5]: 139735675829040

    In [6]: a is b              <-- NOTE
    Out[6]: False

    In [7]: a == b              <-- NOTE
    Out[7]: True

So comparing strings by is is generally incorrect.

-> Fix strings comparision to use == / != everywhere (at least in found
places where string is compared wrt string literal)

We already had similar fix in a8526f4e, but seems the story continues
again.

/cc @alain.takoudjou, @Just1, @lu.xu, @jhuge, @tomo
/reviewed-by @jerome
/reviewed-on !122