Create constants for the EDP job type strings

Currently, string literals are used for the EDP job type values throughout the savanna-api code. We should define constants for these strings and replace the string literal usage with constants.

The string literal job type values are

"MapReduce"
"MapReduce.Streaming"
"Java"
"Pig"
"Hive"

We should probably define constants for subtypes too for consistency and build the above constants with JOB_TYPE + SUBTYPE where applicable. Currently this just applies to the "Streaming" subtype and the "MapReduce.Streaming" job type.

Note that in some places, single quotes are used and in some places double quotes are used, and "Streaming" may be used by itself as a subtype. This makes searching for all the references a little harder.

The places that need to be modified can be found easily with ack or grep:

$ pwd
/home/tmckay/src/savanna/savanna

$ ack "job.type.*[\"\']"