Support cert/private keys as string in connection profile

[marcelloromani]

The connection profile (network.json) supports specifying certs and private keys as a path.

Similarly to what the NodeJS sdk client provides (https://github.com/hyperledger/fabric-sdk-node/blob/master/test/fixtures/profiles/network.json#L49) it would be useful to allow specifying the certificates contents directly as strings, e.g.

      "users": {
        "Admin": {
          "cert_pem": "-----BEGIN CERTIFICATE-----\nMIIB+zCCAaGgAwIBAgIUSFaxTErz/QK2xfiFuHR3km8cNFowCgYIKoZIzj0EAwIw\nWjELMAkGA1UEBhMCVVMxFzAVBgNVBAgTDk5vcn ... n-----END CERTIFICATE-----"
          "private_key_pem": "-----BEGIN PRIVATE KEY-----\nMIGHAgEAM ... 5zIHXyi2yB+hWxJ\n-----END PRIVATE KEY-----"
        }
      }

[...]

      "tlsCACerts": {
        "pem": "-----BEGIN CERTIFICATE-----\nMIIB+zCCAaGgAwIBAgIUSFaxTErz/QK2xfiFuHR3km8cNFowCgYIKoZIzj0EAwIw\nWjELMAkGA1UEBhMCVVMxFzAVBgNVBAgTDk5vcn ... n-----END CERTIFICATE-----"
      },