sync: Support manual authentication to the manifest server

Add two new command line options, -u/--manifest-server-username and
-p/--manifest-server-password, which can be used to specify a username
and password to authenticate to the manifest server when using the
-s/--smart-sync or -t/--smart-tag option.

If -u and -p are not specified when using the -s or -t option, use
authentication credentials from the .netrc file (if there are any).

Authentication credentials from -u/-p or .netrc are not used if the
manifest server specified in the manifest file already includes
credentials.

Change-Id: I6cf9540d28f6cef64c5694e8928cfe367a71d28d
1 file changed