BUG-1953: perform proper string validation

Activates the use ot Patterns to enforce the string value has the format
specified by the yang file. Also fixes mutability of the exposed list,
which may have been attacked, injecting wrong strings.

Change-Id: I32d0ceb836df1f5ed04122a10115f66f75bfa1ec
Signed-off-by: Robert Varga <rovarga@cisco.com>