Changed Project members list style, added scrolls

On current implementation, when a user request to modify project
members, a modal appears with a list of available members, the
buttons may go very far down from screen if the members list is
large.

Co-Authored-By: Joanna H. Huang <joanna.huitzu.huang@gmail.com>
Change-Id: I33912345329cee51b9ce7cd3d658b90d08b142a5
Closes-Bug: #1269910
Cell empty value defaults to - removing redundancy

The default value empty_value for a cell is horizon/tables/base.py
is "-", it is redundant and unnecessary to specify this column
definitions.

Closes-Bug: #1398904
Change-Id: Ica3e8f1559effe6b11f4e81d303758a869acbe2b
Merge "Changed Project members list style, added scrolls"