Allow specifying maximum wait-time for Nailgun

Some tests require Nailgun to be running. For that a server is
started and is waited for 5 seconds. For some systems Nailgun
requires more time to get up and running.

This patch allows users to specify maximum time of waiting Nailgun
to start by setting NAILGUN_START_WAIT_TIME environment variable which
defaults to 5 seconds. Introduced hooks stop tests if Nailgun failed to
start before maximum wait-time passes.

DocImpact
Closes-bug: 1393566
Change-Id: I39d4b312510c34c190ad680c48f1e5792858b140
Merge "Allow specifying maximum wait-time for Nailgun"