Incorrect timezones setting in Login dialog

When enabling time zone selection in the login dialog, some time zones do not behave as expect.

Reproduction path on demo server

1. Enable time zone selection show.timezones in /hippo:configuration/hippo:frontend/login/login/loginPage
2. In the login dialog, select time zone "Etc/GMT+7" and login the CMS.
3. Open a document with time field, click "Set to now" to find out its value.

• Expect: DateTime field should display "Now" value, in this case is 9:53AM (4/8/2016) (GMT+2).
• Actual: DateTime field displays the value 4/8/2016, 0:53AM, which is of the GMT-7.

This bug happens for all Etc/GMT[+|-]? time zones.