snap.yaml app command doesn't allow "=", ":"

[Ian Johnson]

For example, with a statically compiled go program such as vault installed to $SNAP/bin/vault, the following yaml is not allowed by snapd:

```
apps:
  vault:
    command: bin/vault server --config=$VAULT_CONFIG
    daemon: simple
    environment:
      VAULT_CONFIG: "$SNAP_DATA/config/security-secret-store/vault-config.json"
      VAULT_ADDR: "https://localhost:8200"
      VAULT_UI: "true"
    plugs:
      - network
      - network-bind
```

The following message is returned from snapcraft:

error: cannot validate snap "edgexfoundry": invalid definition of application "vault": app description field 'command' contains illegal "bin/vault server --config=$VAULT_CONFIG" (legal: '^[A-Za-z0-9/. _#:$-]*$')

I think that "=" should be added to the list of allowed symbols and that the message could be worded better, something like

invalid definition of application "vault": app description field 'command' is illegal with "bin/vault server --config=$VAULT_CONFIG" (legal: '^[A-Za-z0-9/. _#:$-]*$')

or if it has the ability to highlight exactly what is wrong about the field:

invalid definition of application "vault": app description field 'command' "bin/vault server --config=$VAULT_CONFIG" contains illegal character "=" (legal: '^[A-Za-z0-9/. _#:$-]*$')

or something else because as it is, it doesn't say exactly what is illegal about the specification and it took me a while to see that the "=" is not allowed (which I also think it should be as specifying the value of an option with an "=" is a common thing to do

(also note that yes this is using snapcraft, but I am specifying `adapter: none` in the snapcraft.yaml because it's a statically compiled go program so I don't need the snapcraft wrappers for this)

[Ian Johnson (anonymouse67)]

Additionally, in this same vein it would be very useful to be able to specify arguments with ":" in them, i.e. to specify the address to connect to for another app in the same snap, we need to do this:

```
command: bin/device-random -confdir $SNAP_DATA/config/device-random -profile res --registry $CONSUL_ADDR
    environment:
      CONSUL_ADDR: "consul://localhost:8500"
```

Where ideally we would just be able to do:

```
command: bin/device-random -confdir $SNAP_DATA/config/device-random -profile res --registry=consul://localhost:8500
```