snapcraft validate and sign-build are inconsistent when there are multiple keys

[Leo Arias]

When there are multiple signing keys, snapcraft sign-build shows a wizard-like text and lets you enter a number to select the key you want. snapcraft validate instead uses the default key.

I'm not sure what's the right behaviour, but they should be consistent.

$ snapcraft list-keys
    Name SHA3-384 fingerprint
* default pvqbQVTvSgeMIuUXyr0d_4O_DLQNUDitvmficJ66KYcn_qZKDZcBT6XbgRm7Dcwb
* test 5bsceVZqUkv01S4PxD10guWHQqvCNS7Mwv-I_U089NbcLhC-hnQ78zMGQGr9IZFN

$ snapcraft validate u1test20161026 ubuntu-core=4
Getting details for ubuntu-core
Signing validation ubuntu-core=4

You need a passphrase to unlock the secret key for
user: "default"
4096-bit RSA key, ID D017FEB3, created 2016-01-01

$ snapcraft sign-build *.snap
Select a key:

  Number Name SHA3-384 fingerprint
       1 default pvqbQVTvSgeMIuUXyr0d_4O_DLQNUDitvmficJ66KYcn_qZKDZcBT6XbgRm7Dcwb
       2 test 5bsceVZqUkv01S4PxD10guWHQqvCNS7Mwv-I_U089NbcLhC-hnQ78zMGQGr9IZFN

Key number: 1

You need a passphrase to unlock the secret key for
user: "default"
4096-bit RSA key, ID D017FEB3, created 2016-01-01