CFY-1758 Reuse one token instead of logging in to Salt API several times

SaltRESTManager.log_in was split into two methods to allow creating
a session using existing token.

Token is no longer checked for being valid when logging out. We
were occasionally experiencing a very strange problem: the token
received from Salt API had its start date set to about 1 minute
in the future from the moment of creation.  Strangely, Salt API
accepted this token even before its reported start date, but our
sanity check was catching that situation, so we turned it off.
We also ensured that log_out method will return valid values in
such situations.