Ability to customize "--replSet" via environment variable instead of command

[tianon]

Currently, to modify the value of --replSet, one has to replace the entire image command, ala:

mongodb/local-compose.yml

Line 5 in a862b1d

command: containerpilot mongod --replSet=pilot

        command: containerpilot mongod --replSet=pilot

This is a bit fragile for users who might want to specify not only a custom --replSet, but possibly also other mongod arguments, especially if we end up needing to change the default image command at some point too (for whatever reason).

It'd be nice if this could be specified somehow via environment variable to override the current default value of joyent to avoid these issues (although that does get us into a bit of a catch-22 with TritonDataCenter/containerpilot#126 -- possibly most easily solved via a short wrapper script to run in front of/in place of mongod).

[leoj3n]

I think your env var recommendation has been implemented, which is why I made PR #19.