webadmin: set sub-tab height in proportion with maintab

When webadmin in viewed in small resolution, sub-tab
is taking most of the space. The user has to use
the splitter to reduce the height of the subtab.

Sub-tab height will be set to half of main-tab's height,
and with a maximum of 300PX.

Change-Id: I8cdbb8ac17632e4e9d62a053b89e440f97eeea26
Bug-Url: https://bugzilla.redhat.com/1027178
Signed-off-by: Kanagaraj M <kmayilsa@redhat.com>