rest: extract elements instead of matching

[jkoelker]

If the host has old style VIPs, then the matching will fail, instead
just extract the first two elements regardless of the length.

dcos/dcos#7568

[codecov]

Codecov Report

Merging #199 into master will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #199      +/-   ##
==========================================
+ Coverage   66.02%   66.05%   +0.02%     
==========================================
  Files          55       55              
  Lines        3759     3759              
==========================================
+ Hits         2482     2483       +1     
+ Misses       1277     1276       -1     

Flag	Coverage Δ
#ct	46.02% <ø> (+0.02%)	⬆️
#ct2	46.02% <ø> (+0.02%)	⬆️
#eunit	24.79% <ø> (ø)

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Impacted Files	Coverage Δ
apps/dcos_rest/src/dcos_rest_vips_handler.erl	45.00% <ø> (ø)
apps/dcos_l4lb/src/dcos_l4lb_mgr.erl	78.86% <0.00%> (+0.31%)	⬆️

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