dovetail tool: bugfix about setup dovetail

dovetail tool: bugfix about setup dovetail

1. change the entry_points of setup.cfg into dovetail.run:main
2. after setup the command 'dovetail -d true --testsuite debug' can just be
   successfully executed under folder dovetail/dovetail. That is mainly because
   of the load functions of testcase.py using relative paths.
3. using absolute path instead of relative path can support the command
   successfully executed under any path.

JIRA: DOVETAIL-154

Change-Id: I4a2ad39a1a60e7e63efc55515781d27b823aa894
Signed-off-by: xudan <xudan16@huawei.com>