Update README.md for running cinder tests in tempest

Two changes to README.md are made:
1. Fixed command line for calling cinder-tempest-plugin tests using plugin name
2. Removed usage of deprecated tox command [all-plugin -> all].
https://github.com/openstack/tempest/blob/master/tox.ini#L55

Change-Id: Ic2c88acfba606592dd12677cc70649ff545c1222
1 file changed