MB-8801 ability to specify the password in cbreset_password

also:
- removed duplicate password validation code
- moved reset_admin_password to menelaus_web to reuse the validation code

Change-Id: I15015b934b4e82a3cdc4c1fc182a84b04328f6cd
Reviewed-on: http://review.couchbase.org/28238
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>