[openstack|compute] Allow to use Symbol when specifying the hash of NICs

When use VM with NICs that was introduce in #1669, at this time the key of hash can only use String.
This commit allows to use Symbol as the key of hash.
For example, it is useful when generate hash from yaml.
Merge pull request #2649 from tenforward/openstack_nics_symbol

[openstack|compute] Allow to use Symbol when specifying the hash of NICs