Fix warnings/errors with newer pylint

To help developing Ganeti on newer distributions, let's try to fix
pylint warnings/errors. I'm using pylint from current Debian wheezy:
pylint 0.25.1, astng 0.23.1, common 0.58.0, and we have 3 things that
needs fixing.

First, a really wide "except", with the silencing in the wrong
place. I'm not sure why this doesn't have "except Exception", so let's
add it. However, pylint still complains about "Catching too general
exception", even though we do want to catch both system and our
exception, so let's add a silence for W0703. It's true that we
shouldn't catch KeyboardInterrupt and friends, but that should be
cleaned up on the master branch.

Second, pylint complains about "redefining name builtin tuple",
because we do some pattern matching in the except blocks in
netutils. This seems to be a false positive, but let's clean the code
around this.

And finally, type inference again goes bad, so let's silence E1103
with its "boolean doesn't have 'get' method".

After this, I can run "make lint", and by extension "make
commit-check" on Debian Wheezy, yay! We might be able to bump our
required pylint versions to something not ancient…

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>