Enable non-ascii characters in flavor names

The current implementation provided under commit e6b42d made it so that
flavor names did not have special characters (e.g. *&^%$ etc. except for
-._ and space) however it also limited names to only ascii letters.

This patch makes it so that special characters are still not allowed,
except the ones currently allowed, but it allows the use of non-ascii
characters so that flavor names can be created in locales other than
English.

Change-Id: I056451a5e8b7a49f86e0a11d0011393dc769db34
Closes-bug: #1241625
Merge "Enable non-ascii characters in flavor names"