Don't ignore the HORIZON_CONFIG['user_home'] setting

Dashboard has its own splash, with a hardcoded get_user_home that
igonres HORIZON_CONFIG. This patch makes it use horiozn.get_user_home
which actually checks the user_home setting, and with the default
settings.py, uses dashboard's get_user_home anyways, but allows for
changing that default behavior.

Change-Id: Id2d957f0cd8fbbbbb8f2c053f0c5df4be177e18c
Closes-bug: #1301935
(cherry picked from commit ef2f59f09dd9c27b202a65b38ccf77165f7363a4)