##### SUMMARY
`ansible-galaxy collection install` currently requires the user to specify an install path using the `-p` option. Galaxy currently generates and displays the CLI command for installing a collection. There is no way for Galaxy to determine a value for `-p`, and hard coding a default value into Galaxy makes no sense.

##### ISSUE TYPE
- Bug Report

##### COMPONENT NAME
`ansible-galaxy`

##### ANSIBLE VERSION
<!--- Paste verbatim output from "ansible --version" between quotes -->
```
ansible 2.10.0.dev0
  config file = /etc/ansible/ansible.cfg
  configured module search path = [u'/Users/chouseknecht/.ansible/plugins/modules', u'/usr/share/ansible/plugins/modules']
  ansible python module location = /Users/chouseknecht/projects/ansible/lib/ansible
  executable location = /Users/chouseknecht/.pyenv/versions/venv27/bin/ansible
  python version = 2.7.14 (default, Nov 14 2017, 23:24:24) [GCC 4.2.1 Compatible Apple LLVM 9.0.0 (clang-900.0.38)]
```

roles_path.add_argument('-p', '--roles-path', dest='roles_path', type=opt_help.unfrack_path(pathsep=True),
                                default=C.DEFAULT_ROLES_PATH, action=opt_help.PrependListAction,
                                help='The path to the directory containing your roles. The default is the first '
                                     'writable one configured via DEFAULT_ROLES_PATH: %s ' % default_roles_path)
        
        type_parser = self.parser.add_subparsers(metavar='TYPE', dest='type')
        type_parser.required = True
        
        collection = type_parser.add_parser('collection', help='Manage an Ansible Galaxy collection.')
        collection_parser = collection.add_subparsers(metavar='COLLECTION_ACTION', dest='action')
        collection_parser.required = True
        self.add_init_options(collection_parser, parents=[common, force])
        self.add_build_options(collection_parser, parents=[common, force])
        self.add_publish_options(collection_parser, parents=[common])
        self.add_install_options(collection_parser, parents=[common, force])
        
        role = type_parser.add_parser('role', help='Manage an Ansible Galaxy role.')
        role_parser = role.add_subparsers(metavar='ROLE_ACTION', dest='action')
        role_parser.required = True
        self.add_init_options(role_parser, parents=[common, force, offline])
        self.add_remove_options(role_parser, parents=[common, roles_path])
        self.add_delete_options(role_parser, parents=[common, github])
        self.add_list_options(role_parser, parents=[common, roles_path])
        self.add_search_options(role_parser, parents=[common])
        self.add_import_options(role_parser, parents=[common, github])
        self.add_setup_options(role_parser, parents=[common, roles_path])
        self.add_login_options(role_parser, parents=[common])
        self.add_info_options(role_parser, parents=[common, roles_path, offline])
        self.add_install_options(role_parser, parents=[common, force, roles_path])
    def add_init_options(self, parser, parents=None):