CAS-1386: Fix IV handling for ClearPass in clustered environments

Required in clustered environments where CAS Server B is unable to decrypt ciphertext generated on CAS Server A where the original Initialization Vector remains in a local ConcurrentHashMap.

Patch details:
- Prepends ciphertext with IV (16 bytes); mandatory for proper decryption.
- Derives IV length from Cipher#getBlockSize
- Prepends ciphertext (and IV) with IV length to handle dynamic IV length case.

Affects:
- https://wiki.jasig.org/display/CASUM/ClearPass+and+Multiple+Server+Configurations

Fixes:
- NullPointerException
- BadPaddingException

Reference:
- https://gist.github.com/rdev5/7570750/

JIRA:
- https://issues.jasig.org/browse/CAS-1386