Allow the user to disable the global timeout logic

Allow the user to disable the global timeout logic

The global timeout logic is useful to not allow a test suite
to hang around in a periodic job for 2 weeks or something
like this. However when running the test suites manually. the
global timeout logic tends to be more of a nuisance than a
help, aborting the test suite prematurely, especially when
longer than normal actions are executed in the loops guarded
by the timeout logic.

This change allows the user to redefine a Robot variable
(ENABLE_NETCONF_TEST_TIMEOUT) to tell the suite to not
evaluate any global timeouts. By setting this variable the
user assumes the responsibility of deciding when the job is
running for too long and aborting it if that happens. This
allows the user to run the test suite with the lengthy
actions into completion without messing around with the
timeouts configured in it.

There is also a "global control" Robot variable added
(ENABLE_GLOBAL_TEST_DEADLINES} which disables all global
timeouts. The abovementioned variable disables only the
global deadline for the Netconf scaling tests.

Change-Id: I636379405f335fd45f93e0311afcbea18319778e
Signed-off-by: Jozef Behran <jbehran@cisco.com>