MINOR: sample: mark the truly constant sample fetch keywords as such

MINOR: sample: mark the truly constant sample fetch keywords as such

A number of keywords are really constant and safe to use at config
time. This is the case for str(), int() etc but also env(), hostname(),
nbproc() etc. By extension a few other ones which can be useful to
preset values in a configuration were enabled as well, like data(),
rand() or uuid(). At the moment this doesn't change anything as they
are still only usable from runtime rules.

The "var()" keyword was also marked as const as it can definitely
return stable stuff at boot time.