Fixes #53 - mugc not respecting function_prefix

[jantman]

Description

c7n allows a configurable Lambda function name prefix, which defaults to custodian-. For the policies, this is configured in the actual policy YML. However, for mugc garbage collection, it needs to be explicitly passed in. In the current release, we have mugc hard-coded to the default of custodian-. The result of this, as described in #53, is that manheim-c7n-tools can provision policies with a non-default prefix but mugc keeps trying to clean up the default-prefixed policies.

This PR solves this issue with 3 pieces:

1. A new function_prefix option in manheim-c7n-tools.yml, which defaults to the current and upstream default of custodian-.
2. Modify MugcStep to use that configuration option instead of the hard-coded custodian-.
3. Add a new policy sanity check which causes policygen to fail if any policies are configured with a function-prefix different from what's in the Config object (manheim-c7n-tools.yml). This should prevent issues like #53 where the prefixes differ between function creation and garbage collection.

Testing Done

• Complete unit test coverage
• Manual tests of manheim-c7n-runner -A -s policygen dryrun accountname to verify behavior

[codecov-commenter]

Codecov Report

Merging #54 into master will increase coverage by 0.24%. The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master      #54      +/-   ##
==========================================
+ Coverage   57.73%   57.98%   +0.24%     
==========================================
  Files           8        8              
  Lines        1389     1397       +8     
  Branches      262      264       +2     
==========================================
+ Hits          802      810       +8     
  Misses        586      586              
  Partials        1        1              

Impacted Files	Coverage Δ
manheim_c7n_tools/errorscan.py	0.00% <0.00%> (ø)
manheim_c7n_tools/config.py	100.00% <100.00%> (ø)
manheim_c7n_tools/policygen.py	99.76% <100.00%> (+<0.01%)	:arrow_up:
manheim_c7n_tools/runner.py	100.00% <100.00%> (ø)
manheim_c7n_tools/version.py	100.00% <100.00%> (ø)

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