Refactor Functest docker container

Refactor Functest docker container

1. start.sh has been splitted into 2 scripts:
     - prepare_env.sh : installs functest environment
                        without running any test
     - run_tests.sh : run the Functest tests

2. Add possibility to give the config_functest.yaml
   in the docker run command. So far, it has always
   been used the default in the repo.

3. Possible to give the repos the branch and commit ID
   to be checkout/reset to. This allows versioning control
   for the repos

4. Added -x permissions to shell scripts

JIRA: FUNCTEST-29

Change-Id: If4b779f5baa37531603955db3681e24a5e08251a
Signed-off-by: jose.lausuch <jose.lausuch@ericsson.com>