Bugfix: yardstick env prepare cmd do not support other installer

Bugfix: yardstick env prepare cmd do not support other installer

JIRA: YARDSTICK-629

Currently yardstick env prepare do not support other installer.
I will add follow support:
1. Environment variable already exists: then do not fetch openrc file
and do not add EXTERNAL_NETWORK variable.
2. Already have openrc file in /etc/yardstick/openstack.creds: only
source this file in API.
3.Environment variable not exists and openrc file not in
/etc/yardstick/openstack.creds: fetch openrc file and append
EXTERNAL_NETWORK variable.

Change-Id: I4ce98f2e17ef8e0a0a1c33c3862ca301c53bb6c2
Signed-off-by: chenjiankun <chenjiankun1@huawei.com>