Censor passwords from the HTTP message

A list of possible keywords that may contain the user password
(in a POST request for example) is configured in packetbeat.conf
under [passwords] section:

[passwords]
hide_keywords=[pass=, passwd=, Password=].

The value of the password is replaced by ******. For example pass=secret
is replaced by pass=*****.
Merge branch 'master' of github.com:packetbeat/packetbeat