encrypt passwords and keys

[abarz722]

goal: database must not contain any cleartext passwords (currently mainly fw management api passwords for importing)

• put the encryption key (mainkey) into file in etc/secrets
• to keep it idempotent, upgrade script must first check if a key is already encrypted by trying to decrypt
• all reads to passwords (both in importer and C# parts) must read the mainkey and decrypt the passwords with it, then resume as before
• all pwd writes must add salt and enrypt before writing passwords/keys
• encrypt ldap_connections.ldap_search_user_pwd
• decrypt pwd in importer
• decrypt pwd in autodiscovery (mw)

[tpurschke]

needed for compliance reasons