minor fixes for adding/removing interfaces from network service

[abessiari]

• catch exception in add_interface when interface is not in slice. This happens with a facility port
• avoid key error by first checking for key subnets before update fablib_data
• Tested by creating a slice with a single node, a network service and a facility port. Then I modified the slice by adding/removing a node
  Add/Remove Network Interfaces for Facility Ports #284

[github-actions]

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[coveralls]

Pull Request Test Coverage Report for Build 7779695397

• -6 of 6 (0.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.02%) to 22.197%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
fabrictestbed_extensions/fablib/network_service.py	0	6	0.0%

Totals
Change from base Build 7703994008:	-0.02%
Covered Lines:	1159
Relevant Lines:	4655

---

💛 - Coveralls

[abessiari]

@kthare10

Hi here is the PR as per your request.

[kthare10]

@abessiari - I'll merge this PR. Please consider enabling signed commits for future commits on FABRIC as most of the repositories only allow signed commits.

[kthare10]

Looks good!