Adding registry flag

Allow alternate registry to be supplied, with optional credentials.

If registry is just host:port/path, no credentials will be used
If user:pass@host:port/path, user and pass will be supplied as
credentials.

Change-Id: I8acf67b2916fea059fb306782388910e543b4046
Signed-off-by: beierlm <mark.beierl@canonical.com>