Fix ephemeral mounts

Add all possible ephemeral block mounts during AMI registration

Value for this being if not using directly AWS to launch a server (ergo skipping the assignment).

Prime case is Rightscale: they create their images with 1-4 ephemeral mounts. Due to this on some instance sizes you don't see all the available ephemeral storage and there is no way to add it afterwards.
You could just build the block map as a list and the join it with commas , like this:
```
eph_block_map = ','.join(["ephemeral{idx}=sd{letter}".format(idx=idx, letter=letter) for idx, letter in enumerate(string.ascii_lowercase[1:-1])])
```

This yields from `ephemeral0=sdb` to `ephemeral23=sdy`, though I'm not sure why you don't want `sda` and `sdz` included?