PDI-10716 - Mail job doesn't work with encrypted passwords passed as variables (kettle.properties)

What was done:
1) Added a call to decrypt method at runtime
PDI-10716 - Mail job doesn't work with encrypted passwords passed as variables (kettle.properties)

What was done:
1) Refactored a bit to make it unit-testable
2) Added unit tests for 4 cases

Conflicts:
	engine/src/org/pentaho/di/job/entries/mail/JobEntryMail.java
Merge pull request #246 from kolinus/PDI-10716-2

PDI-10716 - Mail job doesn't work with encrypted passwords passed as variables (kettle.properties)