Finish porting functions to the new client

[migara]

[shinmog]

This is specific to functions on the current structs:

• pango.Client
• pango.Firewall
• pango.Panorama

Other functions in the namespaces are not included in this.

I don't believe that all the old functions belong hanging off of the new client struct, I just think that there should still be a way to invoke those functions in the new framework. For some functions, maybe they belong in specific namespaces. Or perhaps the functions could be added as a new namespace (for example, there are a few functions about config locks. Perhaps that should be in a namespace instead).

[sebastianczech]

Work for that issue was started in #109 , but it's only the beginning:

• pango.XmlApiClient was renamed to pango.Client (it replaces old pango.Client)
• currently not used files were removed from util package

For new pango.Client still there are missed below functions:

• func (c *Client) Commit() {}
• func (c *Client) CommitLocks() {}
• func (c *Client) ConfigLocks() {}
• func (c *Client) ConfigTree() {}
• func (c *Client) LockCommits() {}
• func (c *Client) LockConfig() {}

While implementing all functions for new Client and while creating new package commit, there should be taken into account changes prepared by @pimielowski in below PRs:

• !feature(util/commit_push): Refactor Commit/Push script terraform-panos-ngfw-modules#54
• fix(client): Fix logic on config locks + change funcs name for more obvious  pango#113
• feature(validation): Improve validation option in Panorama pango#111