Introduce sensible default settings for run time

[cyberw]

Some things in Locust require the user to be overly verbose/explicit.

I think we should:

1. When run in headless mode, make run time (-t) default to infinity and user count (-u) default to 1
2. Host (-H / host attribute on the User) should not be mandatory, as the user might be using all absolute URLs, or even a non-HTTP user where a "base host" might not make sense at all.
3. Make the wait_time default to 0 (Make wait_time default to zero (vote up/down for this ticket please :) #1308)

Does anyone else like/not like that idea?

[cyberw]

run time default fixed in #1625

[cyberw]

wait_time default fixed in #1626

making -H non-mandatory (2.) can be fixed later as part of #1591, so we dont need this issue any more

The only part that is unique for this issue is the default for run time, so I changed the name of the ticket.