config: Enable passing a hostname instead of IP

[ansasaki]

For the options 'ip', 'contact_ip', and 'registrar_ip', allow a hostname to be used instead of an IP address.

Fixes #848

[codecov]

Codecov Report

Attention: Patch coverage is 57.14286% with 6 lines in your changes missing coverage. Please review.

Project coverage is 59.72%. Comparing base (2f7b3ad) to head (e535e16).
Report is 58 commits behind head on master.

Files with missing lines	Patch %	Lines
keylime-agent/src/main.rs	0.00%	6 Missing ⚠️

Additional details and impacted files

Flag	Coverage Δ
e2e-testsuite	59.72% <57.14%> (+2.13%)	⬆️
upstream-unit-tests	59.72% <57.14%> (+8.71%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
keylime-agent/src/config.rs	74.56% <100.00%> (-12.94%)	⬇️
keylime-agent/src/main.rs	27.23% <0.00%> (+1.21%)	⬆️

... and 8 files with indirect coverage changes

[THS-on]

LGTM