Store encrypted password

Now password for SSH authentication file is stored as plain text.

This patch fixes it. Already stored password would be replaced to
encrypted ones if config is saved once.

Fix for JENKINS-23165
Fix tests depends on Config class

To use Config class, jenkins instance is needed.

And add PowerMockIgnore annotation to prevent ClassCastException.
Merge pull request #157 from rinrinne/encrypt-pw

Store encrypted password