Add certificate management to nv2 docker plugin

[sajayantony]

Currently adding and removing certificates from the verification list is really error prone.
Providing an ability to add and remove the certificates would improve the user experience

❯ docker nv2 certificates --help
NAME:
   docker nv2 certificates - Manage certificates used for signing and verification

USAGE:
   docker nv2 certificates command [command options] [arguments...]

COMMANDS:
   add         Add certificate to verification list
   list, ls    List cerificates used for verification
   remove, rm  Remove certificate from verification list
   help, h     Shows a list of commands or help for one command

OPTIONS:
   --help, -h  show help (default: false)

❯ docker nv2 certificates ls
/home/sajay/.notary/keys/wabbit-networks.crt
/home/sajay/code/src/github.com/notaryproject/nv2/cmd/nv2/cert/wabbit-networks.crt

❯ docker nv2 certificates add /home/sajay/.notary/keys/wabbit-networks.crt
Added /home/sajay/.notary/keys/wabbit-networks.crt to verification certificates

❯ docker nv2 certificates rm /home/sajay/code/src/github.com/notaryproject/nv2/cmd/nv2/cert/wabbit-networks.crt
Removed /home/sajay/code/src/github.com/notaryproject/nv2/cmd/nv2/cert/wabbit-networks.crt from list of verification certificates

[sajayantony]

Tracked by Uber issue #88