relayer data dir issues
### How {ENV} for the data-dir is determined (flag vs chain-ID)

The relayer's default value for `--data-dir`, as shown in the help, is `{HOME}/.atomicswap/{ENV}`. This left me confused, as I couldn't find the `--env` flag for the relayer. I had to dig through the code to determine that the value of ENV is derived from the chain ID of the ethereum endpoint.

Options to make our tools consistent:
(1) Add an `--env` flag to the relayer and validate that the chain ID of the ethereum endpoint matches the environment. (Current behavior of `swapd`.)
(2) Remove the `--env` flag from `swapd` and derive it from the chain ID of the ethereum endpoint. (Current behavior of the `realyer`.)

### Avoiding `relayer`/`swapd` conflicts

After determining what `{ENV}` was for the `relayer`, I was then confused as to the ETH private key location.  The help says it is in `{DATA_DIR}/eth.key`, which converts to `{HOME}/.atomicswap/{ENV}/eth.key` if you didn't pass a `--data-dir` flag. This is the same location as `swapd`'s key, which had me concerned, as swapd and the relayer can't share the same key for nonce synchronization reasons.  Digging through the code, I realized that the help lied, and that the default data-dir for the relayer is actually `{HOME}/.atomicswap/{ENV}/relayer` making this another inconsistency between our tools.

Here are some possible pairs that would make our data-dir locations consistent between programs:
1. `{HOME}/.atomicswap/{ENV}/relayer`
`{HOME}/.atomicswap/{ENV}/swapd`
1. `{HOME}/.atomicswap/relayer/{ENV}`
`{HOME}/.atomicswap/swapd/{ENV}`
1. `{HOME}/.atomic/{ENV}/swapd`
`{HOME}/.atomic/{ENV}/relayer`
1. `{HOME}/.atomic/swapd/{ENV}`
`{HOME}/.atomic/relayer/{ENV}`