EXOGTN-1284: Incorrect warning message of configured UI data validators

Problem analysis
	- Several messages are duplicated if username has more than 2 illegal characters.
	- Username configuration accepts still uppercase characters by default.
	- In case gatein.validators.{configurationName}.format.message has a point, only the part after the last point is defined as message

Fix description
	- For each invalid condition of username, only add a message
	- Correct behaviour to forbid uppercase characters
	- Get all characters of format.message value
EXOGTN-1284: Remove GroupMembershipValidator in Group Membership Dialog