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: No need to run snf-image-update-helper

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

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