cli: Support auto-completion on options prefixed with "--" (#854473)

Auto complete does not work if the '--' is already written.

Change-Id: I0bb47fa8b0b590678ecaef3d25436d25ca541200
Bug-Url: https://bugzilla.redhat.com/854473
Signed-off-by: Ravi Nori <rnori@redhat.com>