api: fix unsupported dummy cache type

[imeoer]

The dummycache type is missed to handle in config validation:

ERROR [/src/fusedev.rs:595] service mount error: RAFS failed to handle request, Failed to load config: failed to parse configuration information`
ERROR [/src/error.rs:18] Stack:
   0: backtrace::backtrace::trace
   1: backtrace::capture::Backtrace::new

ERROR [/src/error.rs:19] Error:
        Rafs(LoadConfig(Custom { kind: InvalidInput, error: "failed to parse configuration information" }))
        at service/src/fusedev.rs:596
ERROR [src/bin/nydusd/main.rs:525] Failed in starting daemon:
Error: Custom { kind: Other, error: "" }

Fix #1456

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• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
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• I have updated the documentation accordingly.
• I have added tests to cover my changes.

[codecov]

Codecov Report

Merging #1458 (3f89da5) into master (f55d2c9) will decrease coverage by 0.02%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #1458      +/-   ##
==========================================
- Coverage   46.34%   46.33%   -0.02%     
==========================================
  Files         123      123              
  Lines       38896    38896              
  Branches    38896    38896              
==========================================
- Hits        18026    18022       -4     
- Misses      19898    19899       +1     
- Partials      972      975       +3     

Files	Coverage Δ
api/src/config.rs	71.36% <0.00%> (-0.08%)	⬇️

... and 3 files with indirect coverage changes

[adamqqqplay]

LGTM, thanks!