Allow support for different authentication providers

[ahitrin]

At the current moment, we have the only one possibility to set password for an account: the configuration file. This is acceptable in corporate environment, where all passwords may be managed by the one person and used by all other persons. But in single user environment this situation may not be useful enough:

• user may want to store hosts in accounts file, but enter passwords manually
• user may want to use ssh keys instead of passwords
• user may use some kind of password manager and may want to use its API to provide passwords

Therefore, SA should allow user to determine how to provide authentication information for each account.

UPD:

Additional use case:

• in corporate environment, this feature may allow to (partially) split hosts and passwords management. all hosts are stored in one file enabled for all employees, but each password request is being made through external API, which may allow or deny access (and also write access history into the log, like in Logging to the external syslog #33)