Changed connector protocols to use constants
Changed connector protocols to use constants

Originally the protocols in the connector factory method were checked
against strings. This change moves those strings out as constants and
now compares against the constants.

These constants will be used by nova and cinder so the protocol values
aren't hard-coded when being used with os-brick.

Change-Id: I0942c352a8d46fc29565e9555f97f8e9d86cda36
Closes-Bug: #1479355

openstack/os-brick,openstack/os-brick