Fix "Variable not found" in Netconf scaling test

The commit 6acfed01ccc4bf0ce66688bb9d0b51beda5a2265 removed
the ENABLE_GLOBAL_TEST_DEADLINES, causing the Netconf scaling
tests to fail with "Variable ${ENABLE_NETCONF_TEST_TIMEOUT}
not found". Revert that bogus change.

Change-Id: I8b305bbd126aa9fde2294cc5219a502f26067145
Signed-off-by: Jozef Behran <jbehran@cisco.com>