LAVA Timeout for boot is too long

This is for both ubuntu and android of course

I think it would be good to be able to set these independently and expose this parameter through the API so that the default could be overwritten.

Was also reported as part of https://bugs.launchpad.net/lava-scheduler/+bug/874270

For linaro-android, importance should be more than Low, as it affects Gerrit CI integration seriously.

The timeout mainly occur in lava_android_test_install and lava_android_test_run with timeout 3600s in bug 874270, but they should be timeout when 2400s for:
    def run(self, tests, timeout=2400):

but the pexpect code set timeout as "None", I think it will wait until client.proc timeout which is 3600s.
rc = pexpect.run(cmd, timeout=None, logfile=sys.stdout, withexitstatus=True)[1]

It is also suggested to have a timeout specified in job file.

As I commented in https://bugs.launchpad.net/lava-scheduler/+bug/874270, I don't think this place is needed to modify.
And for the test to be executed, we don't know the time it runs. for example, the long time test will run dozen hours.

Even we allow an timeless timeout here, we can't set the timeout to None, we can pass the timeout from the caller to allow a flexible control here, and set the default to -1 but not None.