deploy: Introduce unified @parse() decorator

This is used by certain components to parse the output based on
given regular expression and updates the context attributes based
on the given keys.

Modify get_* methods to get one argument that denotes the identity
(mail, service, cluster) to search for.

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>
deploy: Fix snf-image

snf-image-update-helper is done during post-install phase of
the debian package. Remove it from initialize().

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>