Allow user to customize naming convention

[MacHu-GWU]

Issue #, if available:

#1926

Description of changes:

Replace the hard coded naming convention string template %s-%s-%s (...) with constant variable in constants.py module.

# content of constants.py
DEFAULT_LAMBDA_NAME_PREFIX_CONVENTION = "{app_name}-{chalice_stage}"
DEFAULT_LAMBDA_FUNC_NAME_CONVENTION = "{app_name}-{chalice_stage}-{func_name}"
DEFAULT_LAMBDA_LAYER_NAME_CONVENTION = "{app_name}-{chalice_stage}-{layer_name}"
DEFAULT_LAMBDA_ROLE_NAME_CONVENTION = "{app_name}-{chalice_stage}-{func_name}"
DEFAULT_LAMBDA_DEFAULT_ROLE_NAME_CONVENTION = "{app_name}-{chalice_stage}"
DEFAULT_EVENT_RULE_NAME_CONVENTION = "{app_name}-{chalice_stage}-{resource_name}"

As a result, user can do this to customize the function naming convention

# monkey patch
from chalice import constants
constants.DEFAULT_LAMBDA_FUNC_NAME_CONVENTION = "{chalice_stage}-{app_name}-{func_name}"

from chalice import Chalice

...

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