Change the directories of results and some files to DOVETAIL_HOME

Change the directories of results and some files to DOVETAIL_HOME

JIRA: DOVETAIL-438

1. Need an env variable DOVETAIL_HOME in env_config.sh
2. env_config.sh, pod.yaml and id_rsa should be put in path DOVETAIL_HOME/pre_config
3. the results will also be put in this path
4. no need to give the path of openstack credential file while running a testsuite
5. if running with the code, source the env_config.sh file before running
      source $DOVETAIL_HOME/env_config.sh
      dovetail run --testsuite <testsuite_name>
6. if running with Dovetail docker image
      docker run -it --privileged=true -e DOVETAIL_HOME=<dovetail_home_path>
             -v <dovetail_home_path>:<dovetail_home_path>
             -v /var/run/docker.sock:/var/run/docker.sock
             opnfv/dovetail:latest /bin/bash

Change-Id: Ia23078f27a34e293d0075e49c673930f41069cb6
Signed-off-by: xudan <xudan16@huawei.com>