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>
(cherry picked from commit 106188585a33e10cd910acb16e1d18eba77c8268)
Introduce QNameModule.toString()

Rather than using the default, let's add a ToStringHelper based
toString().

Change-Id: Ib9240564db8f710d36f9281054b5e2825d9790a0
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "Introduce QNameModule.toString()"